SQL-Server Interview Questions
What is RDBMS?
Relational Data Base Management Systems (RDBMS) are database management systems
that maintain data records and indices in tables. Relationships may be created and
maintained across and among the data and tables. In a relational database, relationships
between data items are expressed by means of tables. Interdependencies among these
tables are expressed by data values rather than by pointers. This allows a high degree of
data independence. An RDBMS has the capability to recombine the data items from
different files, providing powerful tools for data usage.
What is normalization?
Database normalization is a data design and organization process applied to data
structures based on rules that help build relational databases. In relational database
design, the process of organizing data to minimize redundancy. Normalization usually
involves dividing a database into two or more tables and defining relationships between
the tables. The objective is to isolate data so that additions, deletions, and modifications
of a field can be made in just one table and then propagated through the rest of the
database via the defined relationships.
What are different normalization forms?
1NF: Eliminate Repeating Groups
Make a separate table for each set of related attributes, and give each table a primary key.
Each field contains at most one value from its attribute domain.
2NF: Eliminate Redundant Data
If an attribute depends on only part of a multi-valued key, remove it to a separate table.
3NF: Eliminate Columns Not Dependent On Key
If attributes do not contribute to a description of the key, remove them to a separate table.
All attributes must be directly dependent on the primary key
BCNF: Boyce-Codd Normal Form
If there are non-trivial dependencies between candidate key attributes, separate them out
into distinct tables.
4NF: Isolate Independent Multiple Relationships
No table may contain two or more 1:n or n:m relationships that are not directly related.
5NF: Isolate Semantically Related Multiple Relationships
There may be practical constrains on information that justify separating logically related
many-to-many relationships.
ONF: Optimal Normal Form
A model limited to only simple (elemental) facts, as expressed in Object Role Model
notation.
DKNF: Domain-Key Normal Form
A model free from all modification anomalies.
Remember, these normalization guidelines are cumulative. For a database to be in 3NF, it
must first fulfill all the criteria of a 2NF and 1NF database.
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What is Stored Procedure?
A stored procedure is a named group of SQL statements that have been previously
created and stored in the server database. Stored procedures accept input parameters so
that a single procedure can be used over the network by several clients using different
input data. And when the procedure is modified, all clients automatically get the new
version. Stored procedures reduce network traffic and improve performance. Stored
procedures can be used to help ensure the integrity of the database.
e.g. sp_helpdb, sp_renamedb, sp_depends etc.
What is Trigger?
A trigger is a SQL procedure that initiates an action when an event (INSERT, DELETE or
UPDATE) occurs. Triggers are stored in and managed by the DBMS.Triggers are used to
maintain the referential integrity of data by changing the data in a systematic fashion. A
trigger cannot be called or executed; the DBMS automatically fires the trigger as a result
of a data modification to the associated table. Triggers can be viewed as similar to stored
procedures in that both consist of procedural logic that is stored at the database level.
Stored procedures, however, are not event-drive and are not attached to a specific table as
triggers are. Stored procedures are explicitly executed by invoking a CALL to the
procedure while triggers are implicitly executed. In addition, triggers can also execute
stored procedures.
Nested Trigger:
A trigger can also contain INSERT, UPDATE and DELETE logic within
itself, so when the trigger is fired because of data modification it can also cause another
data modification, thereby firing another trigger. A trigger that contains data modification
logic within itself is called a nested trigger.
What is View?
A simple view can be thought of as a subset of a table. It can be used for retrieving data,
as well as updating or deleting rows. Rows updated or deleted in the view are updated or
deleted in the table the view was created with. It should also be noted that as data in the
original table changes, so does data in the view, as views are the way to look at part of the
original table. The results of using a view are not permanently stored in the database. The
data accessed through a view is actually constructed using standard T-SQL select
command and can come from one to many different base tables or even other views.
What is Index?
An index is a physical structure containing pointers to the data. Indices are created in an
existing table to locate rows more quickly and efficiently. It is possible to create an index
on one or more columns of a table, and each index is given a name. The users cannot see
the indexes, they are just used to speed up queries. Effective indexes are one of the best
ways to improve performance in a database application. A table scan happens when there
is no index available to help a query. In a table scan SQL Server examines every row in
the table to satisfy the query results. Table scans are sometimes unavoidable, but on large
tables, scans have a terrific impact on performance.
Clustered indexes
define the physical sorting of a database table’s rows in the storage
media. For this reason, each database table may have only one clustered index.
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Non-clustered indexes
are created outside of the database table and contain a sorted list of
references to the table itself.
What is the difference between clustered and a non-clustered index?
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clustered index is a special type of index that reorders the way records in the table are
physically stored. Therefore table can have only one clustered index. The leaf nodes of a
clustered index contain the data pages.
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nonclustered index is a special type of index in which the logical order of the index
does not match the physical stored order of the rows on disk. The leaf node of a
nonclustered index does not consist of the data pages. Instead, the leaf nodes contain
index rows.
What are the different index configurations a table can have?
A table can have one of the following index configurations:
No indexes
A clustered index
A clustered index and many nonclustered indexes
A nonclustered index
Many nonclustered indexes
What is cursors?
Cursor is a database object used by applications to manipulate data in a set on a row-byrow
basis, instead of the typical SQL commands that operate on all the rows in the set at
one time.
In order to work with a cursor we need to perform some steps in the following order:
Declare cursor
Open cursor
Fetch row from the cursor
Process fetched row
Close cursor
Deallocate cursor
What is the use of DBCC commands?
DBCC stands for database consistency checker. We use these commands to check the
consistency of the databases, i.e., maintenance, validation task and status checks.
E.g. DBCC CHECKDB - Ensures that tables in the db and the indexes are correctly
linked.
DBCC CHECKALLOC - To check that all pages in a db are correctly allocated.
DBCC CHECKFILEGROUP - Checks all tables file group for any damage.
What is a Linked Server?
Linked Servers is a concept in SQL Server by which we can add other SQL Server to a
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Group and query both the SQL Server dbs using T-SQL Statements. With a linked server,
you can create very clean, easy to follow, SQL statements that allow remote data to be
retrieved, joined and combined with local data.
Storped Procedure sp_addlinkedserver, sp_addlinkedsrvlogin will be used add new
Linked Server.
What is Collation?
Collation refers to a set of rules that determine how data is sorted and compared.
Character data is sorted using rules that define the correct character sequence, with
options for specifying case-sensitivity, accent marks, kana character types and character
width.
What are different type of Collation Sensitivity?
Case sensitivity
A and a, B and b, etc.
Accent sensitivity
a and á, o and ó, etc.
Kana Sensitivity
When Japanese kana characters Hiragana and Katakana are treated differently, it is called
Kana sensitive.
Width sensitivity
When a single-byte character (half-width) and the same character when represented as a
double-byte character (full-width) are treated differently then it is width sensitive.
What’s the difference between a primary key and a unique key?
Both primary key and unique enforce uniqueness of the column on which they are
defined. But by default primary key creates a clustered index on the column, where are
unique creates a nonclustered index by default. Another major difference is that, primary
key doesn’t allow NULLs, but unique key allows one NULL only.
How to implement one-to-one, one-to-many and many-to-many relationships while
designing tables?
One-to-One relationship can be implemented as a single table and rarely as two tables
with primary and foreign key relationships.
One-to-Many relationships are implemented by splitting the data into two tables with
primary key and foreign key relationships.
Many-to-Many relationships are implemented using a junction table with the keys from
both the tables forming the composite primary key of the junction table.
What is a NOLOCK?
Using the NOLOCK query optimiser hint is generally considered good practice in order
to improve concurrency on a busy system. When the NOLOCK hint is included in a
SELECT statement, no locks are taken when data is read. The result is a Dirty Read,
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which means that another process could be updating the data at the exact time you are
reading it. There are no guarantees that your query will retrieve the most recent data. The
advantage to performance is that your reading of data will not block updates from taking
place, and updates will not block your reading of data. SELECT statements take Shared
(Read) locks. This means that multiple SELECT statements are allowed simultaneous
access, but other processes are blocked from modifying the data. The updates will queue
until all the reads have completed, and reads requested after the update will wait for the
updates to complete. The result to your system is delay(blocking).
What is difference between DELETE & TRUNCATE commands?
Delete command removes the rows from a table based on the condition that we provide
with a WHERE clause. Truncate will actually remove all the rows from a table and there
will be no data in the table after we run the truncate command.
TRUNCATE
TRUNCATE is faster and uses fewer system and transaction log resources than DELETE.
TRUNCATE removes the data by deallocating the data pages used to store the table’s
data, and only the page deallocations are recorded in the transaction log.
TRUNCATE removes all rows from a table, but the table structure and its columns,
constraints, indexes and so on remain. The counter used by an identity for new rows is
reset to the seed for the column.
You cannot use TRUNCATE TABLE on a table referenced by a FOREIGN KEY
constraint.
Because TRUNCATE TABLE is not logged, it cannot activate a trigger.
TRUNCATE can not be Rolled back using logs.
TRUNCATE is DDL Command.
TRUNCATE Resets identity of the table.
DELETE
DELETE removes rows one at a time and records an entry in the transaction log for each
deleted row.
If you want to retain the identity counter, use DELETE instead. If you want to remove
table definition and its data, use the DROP TABLE statement.
DELETE Can be used with or without a WHERE clause
DELETE Activates Triggers.
DELETE Can be Rolled back using logs.
DELETE is DML Command.
DELETE does not reset identity of the table.
Difference between Function and Stored Procedure?
UDF can be used in the SQL statements anywhere in the WHERE/HAVING/SELECT
section where as Stored procedures cannot be.
UDFs that return tables can be treated as another rowset. This can be used in JOINs with
other tables.
Inline UDF’s can be though of as views that take parameters and can be used in JOINs
and other Rowset operations.
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When is the use of UPDATE_STATISTICS command?
This command is basically used when a large processing of data has occurred. If a large
amount of deletions any modification or Bulk Copy into the tables has occurred, it has to
update the indexes to take these changes into account. UPDATE_STATISTICS updates
the indexes on these tables accordingly.
What types of Joins are possible with Sql Server?
Joins are used in queries to explain how different tables are related. Joins also let you
select data from a table depending upon data from another table.
Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are
further classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER
JOINS.
What is the difference between a HAVING CLAUSE and a WHERE CLAUSE?
Specifies a search condition for a group or an aggregate. HAVING can be used only with
the SELECT statement. HAVING is typically used in a GROUP BY clause. When
GROUP BY is not used, HAVING behaves like a WHERE clause. Having Clause is
basically used only with the GROUP BY function in a query. WHERE Clause is applied
to each row before they are part of the GROUP BY function in a query. HAVING criteria
is applied after the the grouping of rows has occurred.
What is sub-query? Explain properties of sub-query.
Sub-queries are often referred to as sub-selects, as they allow a SELECT statement to be
executed arbitrarily within the body of another SQL statement. A sub-query is executed
by enclosing it in a set of parentheses. Sub-queries are generally used to return a single
row as an atomic value, though they may be used to compare values against multiple
rows with the IN keyword.
A subquery is a SELECT statement that is nested within another T-SQL statement. A
subquery SELECT statement if executed independently of the T-SQL statement, in which
it is nested, will return a result set. Meaning a subquery SELECT statement can
standalone and is not depended on the statement in which it is nested. A subquery
SELECT statement can return any number of values, and can be found in, the column list
of a SELECT statement, a FROM, GROUP BY, HAVING, and/or ORDER BY clauses of
a T-SQL statement. A Subquery can also be used as a parameter to a function call.
Basically a subquery can be used anywhere an expression can be used.
Properties of Sub-Query
A subquery must be enclosed in the parenthesis.
A subquery must be put in the right hand of the comparison operator, and
A subquery cannot contain a ORDER-BY clause.
A query can contain more than one sub-queries.
What are types of sub-queries?
Single-row subquery, where the subquery returns only one row.
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Multiple-row subquery, where the subquery returns multiple rows,.and
Multiple column subquery, where the subquery returns multiple columns.
What is SQL Profiler?
SQL Profiler is a graphical tool that allows system administrators to monitor events in an
instance of Microsoft SQL Server. You can capture and save data about each event to a
file or SQL Server table to analyze later. For example, you can monitor a production
environment to see which stored procedures are hampering performance by executing too
slowly.
Use SQL Profiler to monitor only the events in which you are interested. If traces are
becoming too large, you can filter them based on the information you want, so that only a
subset of the event data is collected. Monitoring too many events adds overhead to the
server and the monitoring process and can cause the trace file or trace table to grow very
large, especially when the monitoring process takes place over a long period of time.
What is User Defined Functions?
User-Defined Functions allow to define its own T-SQL functions that can accept 0 or
more parameters and return a single scalar data value or a table data type.
What kind of User-Defined Functions can be created?
There are three types of User-Defined functions in SQL Server 2000 and they are Scalar,
Inline Table-Valued and Multi-statement Table-valued.
Scalar User-Defined Function
A Scalar user-defined function returns one of the scalar data types. Text, ntext, image and
timestamp data types are not supported. These are the type of user-defined functions that
most developers are used to in other programming languages. You pass in 0 to many
parameters and you get a return value.
Inline Table-Value User-Defined Function
An Inline Table-Value user-defined function returns a table data type and is an
exceptional alternative to a view as the user-defined function can pass parameters into a
T-SQL select command and in essence provide us with a parameterized, non-updateable
view of the underlying tables.
Multi-statement Table-Value User-Defined Function
A Multi-Statement Table-Value user-defined function returns a table and is also an
exceptional alternative to a view as the function can support multiple T-SQL statements
to build the final result where the view is limited to a single SELECT statement. Also, the
ability to pass parameters into a T-SQL select command or a group of them gives us the
capability to in essence create a parameterized, non-updateable view of the data in the
underlying tables. Within the create function command you must define the table
structure that is being returned. After creating this type of user-defined function, It can be
used in the FROM clause of a T-SQL command unlike the behavior found when using a
stored procedure which can also return record sets.
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Which TCP/IP port does SQL Server run on? How can it be changed?
SQL Server runs on port 1433. It can be changed from the Network Utility TCP/IP
properties –> Port number.both on client and the server.
What are the authentication modes in SQL Server? How can it be changed?
Windows mode and mixed mode (SQL & Windows).
To change authentication mode in SQL Server
click Start, Programs, Microsoft SQL
Server and click SQL Enterprise Manager to run SQL Enterprise Manager from the
Microsoft SQL Server program group. Select the server then from the Tools menu
select SQL Server Configuration Properties, and choose the Security page.
Where are SQL server users names and passwords are stored in sql server?
They get stored in master db in the sysxlogins table.
Which command using Query Analyzer will give you the version of SQL server and
operating system?
SELECT SERVERPROPERTY('productversion'), SERVERPROPERTY
('productlevel'), SERVERPROPERTY ('edition')
What is SQL server agent?
SQL Server agent plays an important role in the day-to-day tasks of a database
administrator (DBA). It is often overlooked as one of the main tools for SQL Server
management. Its purpose is to ease the implementation of tasks for the DBA, with its fullfunction
scheduling engine, which allows you to schedule your own jobs and scripts.
Can a stored procedure call itself or recursive stored procedure? How many level SP
nesting possible?
Yes. Because Transact-SQL supports recursion, you can write stored procedures that call
themselves. Recursion can be defined as a method of problem solving wherein the
solution is arrived at by repetitively applying it to subsets of the problem. A common
application of recursive logic is to perform numeric computations that lend themselves to
repetitive evaluation by the same processing steps. Stored procedures are nested when
one stored procedure calls another or executes managed code by referencing a CLR
routine, type, or aggregate. You can nest stored procedures and managed code references
up to 32 levels.
What is @@ERROR?
The @@ERROR automatic variable returns the error code of the last Transact-SQL
statement. If there was no error, @@ERROR returns zero. Because @@ERROR is reset
after each Transact-SQL statement, it must be saved to a variable if it is needed to process
it further after checking it.
What is Raiseerror?
Stored procedures report errors to client applications via the RAISERROR command.
RAISERROR doesn’t change the flow of a procedure; it merely displays an error
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message, sets the @@ERROR automatic variable, and optionally writes the message to
the SQL Server error log and the NT application event log.
What is log shipping?
Log shipping is the process of automating the backup of database and transaction log
files on a production SQL server, and then restoring them onto a standby server.
Enterprise Editions only supports log shipping. In log shipping the transactional log
file from one server is automatically updated into the backup database on the other
server. If one server fails, the other server will have the same db can be used this as
the Disaster Recovery plan. The key feature of log shipping is that is will
automatically backup transaction logs throughout the day and automatically restore
them on the standby server at defined interval.
What is the difference between a local and a global variable?
A
local temporary table exists only for the duration of a connection or, if defined inside a
compound statement, for the duration of the compound statement.
A
global temporary table remains in the database permanently, but the rows exist only
within a given connection. When connection are closed, the data in the global temporary
table disappears. However, the table definition remains with the database for access when
database is opened next time.
What command do we use to rename a db?
sp_renamedb ‘oldname’ , ‘newname’
If someone is using db it will not accept sp_renmaedb. In that case first bring db to single
user using sp_dboptions. Use sp_renamedb to rename database. Use sp_dboptions to
bring database to multi user mode.
What is sp_configure commands and set commands?
Use sp_configure to display or change server-level settings. To change database-level
settings, use ALTER DATABASE. To change settings that affect only the current user
session, use the SET statement.
What are the different types of replication? Explain.
The SQL Server 2000-supported replication types are as follows:
·
Transactional
·
Snapshot
·
Merge
Snapshot replication
distributes data exactly as it appears at a specific moment in time
and does not monitor for updates to the data. Snapshot replication is best used as a
method for replicating data that changes infrequently or where the most up-to-date values
(low latency) are not a requirement. When synchronization occurs, the entire snapshot is
generated and sent to Subscribers.
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Transactional replication
, an initial snapshot of data is applied at Subscribers, and then
when data modifications are made at the Publisher, the individual transactions are
captured and propagated to Subscribers.
Merge replication
is the process of distributing data from Publisher to Subscribers,
allowing the Publisher and Subscribers to make updates while connected or disconnected,
and then merging the updates between sites when they are connected.
What are the OS services that the SQL Server installation adds?
MS SQL SERVER SERVICE, SQL AGENT SERVICE, DTC (Distribution transac coordinator)
What are three SQL keywords used to change or set someone’s permissions?
GRANT, DENY, and REVOKE.
What does it mean to have quoted_identifier on? What are the implications of
having it off?
When SET QUOTED_IDENTIFIER is ON, identifiers can be delimited by double
quotation marks, and literals must be delimited by single quotation marks. When SET
QUOTED_IDENTIFIER is OFF, identifiers cannot be quoted and must follow all
Transact-SQL rules for identifiers.
What is the STUFF function and how does it differ from the REPLACE function?
STUFF function to overwrite existing characters. Using this syntax,
STUFF(string_expression, start, length, replacement_characters), string_expression is the
string that will have characters substituted, start is the starting position, length is the
number of characters in the string that are substituted, and replacement_characters are the
new characters interjected into the string.
REPLACE function to replace existing characters of all occurance. Using this syntax
REPLACE(string_expression, search_string, replacement_string), where every incidence
of search_string found in the string_expression will be replaced with replacement_string.
Using query analyzer, name 3 ways to get an accurate count of the number of
records in a table?
SELECT * FROM table1
SELECT COUNT(*) FROM table1
SELECT rows FROM sysindexes WHERE id = OBJECT_ID(table1) AND indid < 2
How to rebuild Master Database?
Shutdown Microsoft SQL Server 2000, and then run Rebuildm.exe. This is located in the
Program Files\Microsoft SQL Server\80\Tools\Binn directory.
In the Rebuild Master dialog box, click Browse.
In the Browse for Folder dialog box, select the \Data folder on the SQL Server 2000
compact disc or in the shared network directory from which SQL Server 2000 was
installed, and then click OK.
Click Settings. In the Collation Settings dialog box, verify or change settings used for the
master database and all other databases.
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Initially, the default collation settings are shown, but these may not match the collation
selected during setup. You can select the same settings used during setup or select new
collation settings. When done, click OK.
In the Rebuild Master dialog box, click Rebuild to start the process.
The Rebuild Master utility reinstalls the master database.
To continue, you may need to stop a server that is running.
Source: http://msdn2.microsoft.com/en-us/library/aa197950(SQL.80).aspx
What is the basic functions for master, msdb, model, tempdb databases?
The
Master database holds information for all databases located on the SQL Server
instance and is the glue that holds the engine together. Because SQL Server cannot start
without a functioning master database, you must administer this database with care.
The
msdb database stores information regarding database backups, SQL Agent
information, DTS packages, SQL Server jobs, and some replication information such as
for log shipping.
The
tempdb holds temporary objects such as global and local temporary tables and stored
procedures.
The
model is essentially a template database used in the creation of any new user
database created in the instance.
What are primary keys and foreign keys?
Primary keys
are the unique identifiers for each row. They must contain unique values
and cannot be null. Due to their importance in relational databases, Primary keys are the
most fundamental of all keys and constraints. A table can have only one Primary key.
Foreign keys
are both a method of ensuring data integrity and a manifestation of the
relationship between tables.
What is data integrity? Explain constraints?
Data integrity is an important feature in SQL Server. When used properly, it ensures that
data is accurate, correct, and valid. It also acts as a trap for otherwise undetectable bugs
within applications.
A
PRIMARY KEY constraint is a unique identifier for a row within a database table.
Every table should have a primary key constraint to uniquely identify each row and
only one primary key constraint can be created for each table. The primary key
constraints are used to enforce entity integrity.
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UNIQUE constraint enforces the uniqueness of the values in a set of columns, so
no duplicate values are entered. The unique key constraints are used to enforce
entity integrity as the primary key constraints.
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FOREIGN KEY constraint prevents any actions that would destroy links between
tables with the corresponding data values. A foreign key in one table points to a
primary key in another table. Foreign keys prevent actions that would leave rows
with foreign key values when there are no primary keys with that value. The foreign
key constraints are used to enforce referential integrity.
A
CHECK constraint is used to limit the values that can be placed in a column. The
check constraints are used to enforce domain integrity.
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A
NOT NULL constraint enforces that the column will not accept null values. The not
null constraints are used to enforce domain integrity, as the check constraints.
What are the properties of the Relational tables?
Relational tables have six properties:
·
Values are atomic.
·
Column values are of the same kind.
·
Each row is unique.
·
The sequence of columns is insignificant.
·
The sequence of rows is insignificant.
·
Each column must have a unique name.
What is De-normalization?
De-normalization is the process of attempting to optimize the performance of a database
by adding redundant data. It is sometimes necessary because current DBMSs implement
the relational model poorly. A true relational DBMS would allow for a fully normalized
database at the logical level, while providing physical storage of data that is tuned for
high performance. De-normalization is a technique to move from higher to lower normal
forms of database modeling in order to speed up database access.
How to get @@error and @@rowcount at the same time?
If @@Rowcount is checked after Error checking statement then it will have 0 as the
value of @@Recordcount as it would have been reset.
And if @@Recordcount is checked before the error-checking statement then @@Error
would get reset. To get @@error and @@rowcount at the same time do both in same
statement and store them in local variable. SELECT @RC = @@ROWCOUNT, @ER =
@@ERROR
What is Identity?
Identity (or AutoNumber) is a column that automatically generates numeric values. A
start and increment value can be set, but most DBA leave these at 1. A GUID column also
generates numbers, the value of this cannot be controled. Identity/GUID columns do not
need to be indexed.
What is a Scheduled Jobs or What is a Scheduled Tasks?
Scheduled tasks let user automate processes that run on regular or predictable cycles.
User can schedule administrative tasks, such as cube processing, to run during times of
slow business activity. User can also determine the order in which tasks run by creating
job steps within a SQL Server Agent job. E.g. Back up database, Update Stats of Tables.
Job steps give user control over flow of execution. If one job fails, user can configure
SQL Server Agent to continue to run the remaining tasks or to stop execution.
What is a table called, if it does not have neither Cluster nor Non-cluster Index?
What is it used for?
Unindexed table or
Heap. Microsoft Press Books and Book On Line (BOL) refers it as
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Heap.
A heap is a table that does not have a clustered index and, therefore, the pages are not
linked by pointers. The IAM pages are the only structures that link the pages in a table
together.
Unindexed tables are good for fast storing of data. Many times it is better to drop all
indexes from table and than do bulk of inserts and to restore those indexes after that.
What is BCP? When does it used?
BulkCopy is a tool used to copy huge amount of data from tables and views. BCP does
not copy the structures same as source to destination.
How do you load large data to the SQL server database?
BulkCopy is a tool used to copy huge amount of data from tables. BULK INSERT
command helps to Imports a data file into a database table or view in a user-specified
format.
Can we rewrite subqueries into simple select statements or with joins?
Subqueries can often be re-written to use a standard outer join, resulting in faster
performance. As we may know, an outer join uses the plus sign (+) operator to tell the
database to return all non-matching rows with NULL values. Hence we combine the
outer join with a NULL test in the WHERE clause to reproduce the result set without
using a sub-query.
Can SQL Servers linked to other servers like Oracle?
SQL Server can be lined to any server provided it has OLE-DB provider from Microsoft
to allow a link. E.g. Oracle has a OLE-DB provider for oracle that Microsoft provides to
add it as linked server to SQL Server group.
How to know which index a table is using?
SELECT table_name,index_name FROM user_constraints
How to copy the tables, schema and views from one SQL server to another?
Microsoft SQL Server 2000 Data Transformation Services (DTS) is a set of graphical
tools and programmable objects that lets user extract, transform, and consolidate data
from disparate sources into single or multiple destinations.
What is Self Join?
This is a particular case when one table joins to itself, with one or two aliases to avoid
confusion. A self join can be of any type, as long as the joined tables are the same. A self
join is rather unique in that it involves a relationship with only one table. The common
example is when company have a hierarchal reporting structure whereby one member of
staff reports to another.
What is Cross Join?
A cross join that does not have a WHERE clause produces the Cartesian product of the
tables involved in the join. The size of a Cartesian product result set is the number of
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rows in the first table multiplied by the number of rows in the second table. The common
example is when company wants to combine each product with a pricing table to analyze
each product at each price.
Which virtual table does a trigger use?
Inserted and Deleted.
List few advantages of Stored Procedure.
·
Stored procedure can reduced network traffic and latency, boosting application
performance.
·
Stored procedure execution plans can be reused, staying cached in SQL Server’s
memory, reducing server overhead.
·
Stored procedures help promote code reuse.
·
Stored procedures can encapsulate logic. You can change stored procedure code
without affecting clients.
·
Stored procedures provide better security to your data.
What is DataWarehousing?
·
Subject-oriented, meaning that the data in the database is organized so that all
the data elements relating to the same real-world event or object are linked
together;
·
Time-variant, meaning that the changes to the data in the database are tracked
and recorded so that reports can be produced showing changes over time;
·
Non-volatile, meaning that data in the database is never over-written or deleted,
once committed, the data is static, read-only, but retained for future reporting;
·
Integrated, meaning that the database contains data from most or all of an
organization’s operational applications, and that this data is made consistent.
What is OLTP(OnLine Transaction Processing)?
In OLTP - online transaction processing systems relational database design use the
discipline of data modeling and generally follow the Codd rules of data normalization in
order to ensure absolute data integrity. Using these rules complex information is broken
down into its most simple structures (a table) where all of the individual atomic level
elements relate to each other and satisfy the normalization rules.
How do SQL server 2000 and XML linked? Can XML be used to access data?
FOR XML (ROW, AUTO, EXPLICIT)
You can execute SQL queries against existing relational databases to return results as
XML rather than standard rowsets. These queries can be executed directly or from within
stored procedures. To retrieve XML results, use the FOR XML clause of the SELECT
statement and specify an XML mode of RAW, AUTO, or EXPLICIT.
OPENXML
OPENXML is a Transact-SQL keyword that provides a relational/rowset view over an in-
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memory XML document. OPENXML is a rowset provider similar to a table or a view.
OPENXML provides a way to access XML data within the Transact-SQL context by
transferring data from an XML document into the relational tables. Thus, OPENXML
allows you to manage an XML document and its interaction with the relational
environment.
What is an execution plan? When would you use it? How would you view the
execution plan?
An execution plan is basically a road map that graphically or textually shows the data
retrieval methods chosen by the SQL Server query optimizer for a stored procedure or adhoc
query and is a very useful tool for a developer to understand the performance
characteristics of a query or stored procedure since the plan is the one that SQL Server
will place in its cache and use to execute the stored procedure or query. From within
Query Analyzer is an option called “Show Execution Plan” (located on the Query dropdown
menu). If this option is turned on it will display query execution plan in separate
window when query is ran again.
How do you implement one-to-one, one-to-many and many-to-many relationships
while designing tables?
One-to-One relationship can be implemented as a single table and rarely as two tables
with primary and foreign key relationships. One-to-Many relationships are implemented
by splitting the data into two tables with primary key and foreign key relationships.
Many-to-Many relationships are implemented using a junction table with the keys from
both the tables forming the composite primary key of the junction table. It will be a good
idea to read up a database designing fundamentals text book.
What’s the difference between a primary key and a unique key?
Both primary key and unique enforce uniqueness of the column on which they are
defined. But by default primary key creates a clustered index on the column, where are
unique creates a nonclustered index by default. Another major difference is that, primary
key doesn’t allow NULLs, but unique key allows one NULL only.
What are user defined datatypes and when you should go for them?
User defined datatypes let you extend the base SQL Server datatypes by providing a
descriptive name, and format to the database. Take for example, in your database, there is
a column called Flight_Num which appears in many tables. In all these tables it should be
varchar(8). In this case you could create a user defined datatype called Flight_num_type
of varchar(8) and use it across all your tables. See sp_addtype, sp_droptype in books
online.
What is bit datatype and what’s the information that can be stored inside a bit
column?
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Bit datatype is used to store boolean information like 1 or 0 (true or false). Untill SQL
Server 6.5 bit datatype could hold either a 1 or 0 and there was no support for NULL. But
from SQL Server 7.0 onwards, bit datatype can represent a third state, which is NULL.
Define candidate key, alternate key, composite key.
A candidate key is one that can identify each row of a table uniquely. Generally a
candidate key becomes the primary key of the table. If the table has more than one
candidate key, one of them will become the primary key, and the rest are called alternate
keys. A key formed by combining at least two or more columns is called composite key.
What are defaults? Is there a column to which a default can’t be bound?
A default is a value that will be used by a column, if no value is supplied to that column
while inserting data. IDENTITY columns and timestamp columns can’t have defaults
bound to them. See CREATE DEFAULT in books online.
What is a transaction and what are ACID properties?
A transaction is a logical unit of work in which, all the steps must be performed or none.
ACID stands for Atomicity, Consistency, Isolation, Durability. These are the properties of
a transaction. For more information and explanation of these properties, see SQL Server
books online or any RDBMS fundamentals text book. Explain different isolation levels
An isolation level determines the degree of isolation of data between concurrent
transactions. The default SQL Server isolation level is Read Committed. Here are the
other isolation levels (in the ascending order of isolation): Read Uncommitted, Read
Committed, Repeatable Read, Serializable. See SQL Server books online for an
explanation of the isolation levels. Be sure to read about SET TRANSACTION
ISOLATION LEVEL, which lets you customize the isolation level at the connection
level. Read Committed - A transaction operating at the Read Committed level cannot see
changes made by other transactions until those transactions are committed. At this level
of isolation, dirty reads are not possible but nonrepeatable reads and phantoms are
possible. Read Uncommitted - A transaction operating at the Read Uncommitted level
can see uncommitted changes made by other transactions. At this level of isolation, dirty
reads, nonrepeatable reads, and phantoms are all possible. Repeatable Read - A
transaction operating at the Repeatable Read level is guaranteed not to see any changes
made by other transactions in values it has already read. At this level of isolation, dirty
reads and nonrepeatable reads are not possible but phantoms are possible. Serializable - A
transaction operating at the Serializable level guarantees that all concurrent transactions
interact only in ways that produce the same effect as if each transaction were entirely
executed one after the other. At this isolation level, dirty reads, nonrepeatable reads, and
phantoms are not possible.
CREATE INDEX myIndex ON myTable(myColumn)What type of Index will get
created after executing the above statement?
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Non-clustered index. Important thing to note: By default a clustered index gets created on
the primary key, unless specified otherwise.
What’s the maximum size of a row?
8060 bytes. Don’t be surprised with questions like ‘what is the maximum number of
columns per table’. 1024 columns per table. Check out SQL Server books online for the
page titled: "Maximum Capacity Specifications". Explain Active/Active and
Active/Passive cluster configurations Hopefully you have experience setting up cluster
servers. But if you don’t, at least be familiar with the way clustering works and the two
clusterning configurations Active/Active and Active/Passive. SQL Server books online
has enough information on this topic and there is a good white paper available on
Microsoft site. Explain the architecture of SQL Server This is a very important question
and you better be able to answer it if consider yourself a DBA. SQL Server books online
is the best place to read about SQL Server architecture. Read up the chapter dedicated to
SQL Server Architecture.
What is lock escalation?
Lock escalation is the process of converting a lot of low level locks (like row locks, page
locks) into higher level locks (like table locks). Every lock is a memory structure too
many locks would mean, more memory being occupied by locks. To prevent this from
happening, SQL Server escalates the many fine-grain locks to fewer coarse-grain locks.
Lock escalation threshold was definable in SQL Server 6.5, but from SQL Server 7.0
onwards it’s dynamically managed by SQL Server.
What’s the difference between DELETE TABLE and TRUNCATE TABLE
commands?
DELETE TABLE is a logged operation, so the deletion of each row gets logged in the
transaction log, which makes it slow. TRUNCATE TABLE also deletes all the rows in a
table, but it won’t log the deletion of each row, instead it logs the deallocation of the data
pages of the table, which makes it faster. Of course, TRUNCATE TABLE can be rolled
back. TRUNCATE TABLE is functionally identical to DELETE statement with no
WHERE clause: both remove all rows in the table. But TRUNCATE TABLE is faster and
uses fewer system and transaction log resources than DELETE. The DELETE statement
removes rows one at a time and records an entry in the transaction log for each deleted
row. TRUNCATE TABLE removes the data by deallocating the data pages used to store
the table’s data, and only the page deallocations are recorded in the transaction log.
TRUNCATE TABLE removes all rows from a table, but the table structure and its
columns, constraints, indexes and so on remain. The counter used by an identity for new
rows is reset to the seed for the column. If you want to retain the identity counter, use
DELETE instead. If you want to remove table definition and its data, use the DROP
TABLE statement. You cannot use TRUNCATE TABLE on a table referenced by a
FOREIGN KEY constraint; instead, use DELETE statement without a WHERE clause.
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Because TRUNCATE TABLE is not logged, it cannot activate a trigger. TRUNCATE
TABLE may not be used on tables participating in an indexed view
Explain the storage models of OLAP
Check out MOLAP, ROLAP and HOLAP in SQL Server books online for more
infomation.
What are the new features introduced in SQL Server 2000 (or the latest release of
SQL Server at the time of your interview)? What changed between the previous
version of SQL Server and the current version?
This question is generally asked to see how current is your knowledge. Generally there is
a section in the beginning of the books online titled "What’s New", which has all such
information. Of course, reading just that is not enough, you should have tried those things
to better answer the questions. Also check out the section titled "Backward
Compatibility" in books online which talks about the changes that have taken place in the
new version.
What are constraints? Explain different types of constraints.
Constraints enable the RDBMS enforce the integrity of the database automatically,
without needing you to create triggers, rule or defaults. Types of constraints: NOT NULL,
CHECK, UNIQUE, PRIMARY KEY, FOREIGN KEY. For an explanation of these
constraints see books online for the pages titled: "Constraints" and "CREATE TABLE",
"ALTER TABLE"
What is an index? What are the types of indexes? How many clustered indexes can
be created on a table? I create a separate index on each column of a table. What are
the advantages and disadvantages of this approach?
Indexes in SQL Server are similar to the indexes in books. They help SQL Server retrieve
the data quicker. Indexes are of two types. Clustered indexes and non-clustered indexes.
When you create a clustered index on a table, all the rows in the table are stored in the
order of the clustered index key. So, there can be only one clustered index per table. Nonclustered
indexes have their own storage separate from the table data storage. Nonclustered
indexes are stored as B-tree structures (so do clustered indexes), with the leaf
level nodes having the index key and it’s row locater. The row located could be the RID
or the Clustered index key, depending up on the absence or presence of clustered index
on the table. If you create an index on each column of a table, it improves the query
performance, as the query optimizer can choose from all the existing indexes to come up
with an efficient execution plan. At the same t ime, data modification operations (such as
INSERT, UPDATE, DELETE) will become slow, as every time data changes in the table,
all the indexes need to be updated. Another disadvantage is that, indexes need disk space,
the more indexes you have, more disk space is used.
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What is RAID and what are different types of RAID configurations?
RAID stands for Redundant Array of Inexpensive Disks, used to provide fault tolerance
to database servers. There are six RAID levels 0 through 5 offering different levels of
performance, fault tolerance. MSDN has some information about RAID levels and for
detailed information, check out the RAID advisory board’s homepage
What are the steps you will take to improve performance of a poor performing
query?
This is a very open ended question and there could be a lot of reasons behind the poor
performance of a query. But some general issues that you could talk about would be: No
indexes, table scans, missing or out of date statistics, blocking, excess recompilations of
stored procedures, procedures and triggers without SET NOCOUNT ON, poorly written
query with unnecessarily complicated joins, too much normalization, excess usage of
cursors and temporary tables. Some of the tools/ways that help you troubleshooting
performance problems are: SET SHOWPLAN_ALL ON, SET SHOWPLAN_TEXT ON,
SET STATISTICS IO ON, SQL Server Profiler, Windows NT /2000 Performance
monitor, Graphical execution plan in Query Analyzer. Download the white paper on
performance tuning SQL Server from Microsoft web site. Don’t forget to check out sqlserver-
performance.com
What are the steps you will take, if you are tasked with securing an SQL Server?
Again this is another open ended question. Here are some things you could talk about:
Preferring NT authentication, using server, databse and application roles to control access
to the data, securing the physical database files using NTFS permissions, using an
unguessable SA password, restricting physical access to the SQL Server, renaming the
Administrator account on the SQL Server computer, disabling the Guest account,
enabling auditing, using multiprotocol encryption, setting up SSL, setting up firewalls,
isolating SQL Server from the web server etc. Read the white paper on SQL Server
security from Microsoft website. Also check out My SQL Server security best practices
What is a deadlock and what is a live lock? How will you go about resolving
deadlocks?
Deadlock is a situation when two processes, each having a lock on one piece of data,
attempt to acquire a lock on the other’s piece. Each process would wait indefinitely for
the other to release the lock, unless one of the user processes is terminated. SQL Server
detects deadlocks and terminates one user’s process. A livelock is one, where a request
for an exclusive lock is repeatedly denied because a series of overlapping shared locks
keeps interfering. SQL Server detects the situation after four denials and refuses further
shared locks. A livelock also occurs when read transactions monopolize a table or page,
forcing a write transaction to wait indefinitely. Check out SET DEADLOCK_PRIORITY
and "Minimizing Deadlocks" in SQL Server books online. Also check out the article
Q169960 from Microsoft knowledge base.
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What is blocking and how would you troubleshoot it?
Blocking happens when one connection from an application holds a lock and a second
connection requires a conflicting lock type. This forces the second connection to wait,
blocked on the first. Read up the following topics in SQL Server books online:
Understanding and avoiding blocking, Coding efficient transactions. Explain CREATE
DATABASE syntax Many of us are used to creating databases from the Enterprise
Manager or by just issuing the command: CREATE DATABAE MyDB.
But what if you have to create a database with two filegroups, one on drive C and
the other on drive D with log on drive E with an initial size of 600 MB and with a
growth factor of 15%?
That’s why being a DBA you should be familiar with the CREATE DATABASE syntax.
Check out SQL Server books online for more information.
How to restart SQL Server in single user mode? How to start SQL Server in
minimal configuration mode?
SQL Server can be started from command line, using the SQLSERVR.EXE. This EXE
has some very important parameters with which a DBA should be familiar with. -m is
used for starting SQL Server in single user mode and -f is used to start the SQL Server in
minimal configuration mode. Check out SQL Server books online for more parameters
and their explanations.
As a part of your job, what are the DBCC commands that you commonly use for
database maintenance?
DBCC CHECKDB, DBCC CHECKTABLE, DBCC CHECKCATALOG, DBCC
CHECKALLOC, DBCC SHOWCONTIG, DBCC SHRINKDATABASE, DBCC
SHRINKFILE etc. But there are a whole load of DBCC commands which are very useful
for DBAs. Check out SQL Server books online for more information.
What are statistics, under what circumstances they go out of date, how do you
update them?
Statistics determine the selectivity of the indexes. If an indexed column has unique values
then the selectivity of that index is more, as opposed to an index with non-unique values.
Query optimizer uses these indexes in determining whether to choose an index or not
while executing a query. Some situations under which you should update statistics: 1) If
there is significant change in the key values in the index 2) If a large amount of data in an
indexed column has been added, changed, or removed (that is, if the distribution of key
values has changed), or the table has been truncated using the TRUNCATE TABLE
statement and then repopulated 3) Database is upgraded from a previous version. Look up
SQL Server books online for the following commands: UPDATE STATISTICS,
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STATS_DATE, DBCC SHOW_STATISTICS, CREATE STATISTICS, DROP
STATISTICS, sp_autostats, sp_createstats, sp_updatestats
What are the different ways of moving data/databases between servers and
databases in SQL Server?
There are lots of options available, you have to choose your option depending upon your
requirements. Some of the options you have are: BACKUP/RESTORE, dettaching and
attaching databases, replication, DTS, BCP, logshipping, INSERT…SELECT,
SELECT…INTO, creating INSERT scripts to generate data.
Explain different types of BACKUPs avaialabe in SQL Server? Given a particular
scenario, how would you go about choosing a backup plan?
Types of backups you can create in SQL Sever 7.0+ are Full database backup, differential
database backup, transaction log backup, filegroup backup. Check out the BACKUP and
RESTORE commands in SQL Server books online. Be prepared to write the commands
in your interview. Books online also has information on detailed backup/restore
architecture and when one should go for a particular kind of backup.
What is database replication? What are the different types of replication you can set
up in SQL Server?
Replication is the process of copying/moving data between databases on the same or
different servers. SQL Server supports the following types of replication scenarios: ?
Snapshot replication ? Transactional replication (with immediate updating subscribers,
with queued updating subscribers) ? Merge replication See SQL Server books online for
indepth coverage on replication. Be prepared to explain how different replication agents
function, what are the main system tables used in replication etc.
How to determine the service pack currently installed on SQL Server?
The global variable @@Version stores the build number of the sqlservr.exe, which is
used to determine the service pack installed. To know more about this process visit SQL
Server service packs and versions.
What are cursors? Explain different types of cursors. What are the disadvantages of
cursors? How can you avoid cursors?
o
Cursors allow row-by-row processing of the resultsets. Types of cursors:
Static, Dynamic, Forward-only, Keyset-driven. See books online for more
information. Disadvantages of cursors: Each time you fetch a row from the
cursor, it results in a network roundtrip, where as a normal SELECT query
makes only one roundtrip, however large the resultset is. Cursors are also
costly because they require more resources and temporary storage (results
in more IO operations). Further, there are restrictions on the SELECT
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statements that can be used with some types of cursors. Most of the times,
set based operations can be used instead of cursors. Here is an example: If
you have to give a flat hike to your employees using the following criteria:
Salary between 30000 and 40000 — 5000 hike Salary between 40000 and
55000 — 7000 hike Salary between 55000 and 65000 — 9000 hike. In
this situation many developers tend to use a cursor, determine each
employee’s salary and update his salary according to the above formula.
But the same can be achieved by multiple update statements or can be
combined in a single UPDATE statement as shown below:
o
UPDATE tbl_emp SET salary = CASE WHEN salary
BETWEEN 30000 AND 40000 THEN salary + 5000 WHEN
salary BETWEEN 40000 AND 55000 THEN salary + 7000
WHEN salary BETWEEN 55000 AND 65000 THEN salary +
10000 END
o
Another situation in which developers tend to use cursors: You need to call
a stored procedure when a column in a particular row meets certain
condition. You don’t have to use cursors for this. This can be achieved
using WHILE loop, as long as there is a unique key to identify each row.
For examples of using WHILE loop for row by row processing, check out
the ‘My code library’ section of my site or search for WHILE. Write down
the general syntax for a SELECT statements covering all the options.
Here’s the basic syntax: (Also checkout SELECT in books online for
advanced syntax).
o
SELECT select_list [INTO new_table_] FROM
table_source [WHERE search_condition] [GROUP BY
group_by_expression] [HAVING search_condition]
[ORDER BY order_expression [ASC | DESC] ]
What is a join and explain different types of joins.
Joins are used in queries to explain how different tables are related. Joins also let you
select data from a table depending upon data from another table. Types of joins: INNER
JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further classified as LEFT
OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS. For more
information see pages from books online titled: "Join Fundamentals" and "Using Joins".
Can you have a nested transaction?
Yes, very much. Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN and
@@TRANCOUNT
What is an extended stored procedure? Can you instantiate a COM object by using
T-SQL?
An extended stored procedure is a function within a DLL (written in a programming
language like C, C++ using Open Data Services (ODS) API) that can be called from T-
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SQL, just the way we call normal stored procedures using the EXEC statement. See
books online to learn how to create extended stored procedures and how to add them to
SQL Server. Yes, you can instantiate a COM (written in languages like VB, VC++) object
from T-SQL by using sp_OACreate stored procedure. Also see books online for
sp_OAMethod, sp_OAGetProperty, sp_OASetProperty, sp_OADestroy. For an example
of creating a COM object in VB and calling it from T-SQL, see ‘My code library’ section
of this site.
What is the system function to get the current user’s user id?
USER_ID(). Also check out other system functions like USER_NAME(),
SYSTEM_USER, SESSION_USER, CURRENT_USER, USER, SUSER_SID(),
HOST_NAME().
What are triggers? How many triggers you can have on a table? How to invoke a
trigger on demand?
Triggers are special kind of stored procedures that get executed automatically when an
INSERT, UPDATE or DELETE operation takes place on a table. In SQL Server 6.5 you
could define only 3 triggers per table, one for INSERT, one for UPDATE and one for
DELETE. From SQL Server 7.0 onwards, this restriction is gone, and you could create
multiple triggers per each action. But in 7.0 there’s no way to control the order in which
the triggers fire. In SQL Server 2000 you could specify which trigger fires first or fires
last using sp_settriggerorder. Triggers can’t be invoked on demand. They get triggered
only when an associated action (INSERT, UPDATE, DELETE) happens on the table on
which they are defined. Triggers are generally used to implement business rules, auditing.
Triggers can also be used to extend the referential integrity checks, but wherever
possible, use constraints for this purpose, instead of triggers, as constraints are much
faster. Till SQL Server 7.0, triggers fire only after the data modification operation
happens. So in a way, they are called post triggers. But in SQL Server 2000 you could
create pre triggers also. Search SQL Server 2000 books online for INSTEAD OF triggers.
Also check out books online for ‘inserted table’, ‘deleted table’ and
COLUMNS_UPDATED()
There is a trigger defined for INSERT operations on a table, in an OLTP system.
The trigger is written to instantiate a COM object and pass the newly insterted rows
to it for some custom processing. What do you think of this implementation? Can
this be implemented better?
Instantiating COM objects is a time consuming process and since you are doing it from
within a trigger, it slows down the data insertion process. Same is the case with sending
emails from triggers. This scenario can be better implemented by logging all the
necessary data into a separate table, and have a job which periodically checks this table
and does the needful.
What is a self join? Explain it with an example.
23
o
Self join is just like any other join, except that two instances of the same
table will be joined in the query. Here is an example: Employees table
which contains rows for normal employees as well as managers. So, to
find out the managers of all the employees, you need a self join.
o
CREATE TABLE emp ( empid int, mgrid int, empname
char(10) )
o
INSERT emp SELECT 1,2,’Vyas’ INSERT emp SELECT
2,3,’Mohan’ INSERT emp SELECT 3,NULL,’Shobha’
INSERT emp SELECT 4,2,’Shridhar’ INSERT emp
SELECT 5,2,’Sourabh’
o
SELECT t1.empname [Employee], t2.empname
[Manager] FROM emp t1, emp t2 WHERE t1.mgrid =
t2.empid Here’s an advanced query using a LEFT
OUTER JOIN that even returns the employees
without managers (super bosses)
o
SELECT t1.empname [Employee],
COALESCE(t2.empname, ‘No manager’) [Manager] FROM
emp t1 LEFT OUTER JOIN emp t2 ON t1.mgrid =
t2.empid
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