Database links are created to establish communication between different databases or different environments such as development, test and production of the same database. The database links are usually designed to be read-only to access other database information . They are also useful when you want to copy production data into test environment for testing.
Q. What is Oracle's data dictionary used for?
Data dictionary in Oracle contains information about all database objects such as tables, triggers, stored procedures, functions, indexes, constraints, views, users, roles, monitoring information, etc.
Q. Which data dictionary objects are used to retrieve the information about the following objects from a given schema?
1) tables
2) views
3) triggers
4) procedures
5) constraints
6) all of the above mentioned objects
The objects used are:
a> user_tables or tabs
b> user_views
c> user_triggers
d> user_procedures
e> user_constraints
f> user_objects
fferent SQL queries in the same PL/SQL program vs. design time declared explicit cursors with an association to only one query.
Q. You want to view top 50 rows from Oracle table. How do I this?
Use ROWNUM, the pseudo column in where clause as follows:
Where rownum < 51
After complete execution of query and before displaying output of SQL query to the user oracle internally assigns sequential numbers to each row in the output. These numbers are held in the hidden column or pseudo column that is a ROWNUM column. Now it is so simple to apply the above logical condition, as you would have done to any other column of the table.
Q. How do you reference column values in BEFORE and AFTER insert and delete triggers?
The BEFORE and AFTER insert triggers can reference column values by new collection using keyword “:new.column name”. The before and after delete triggers can reference column values by old collection using keyword “:old. column name”.
Q. Can you change the inserted value in one of the columns in AFTER insert trigger code?
This is not possible as the column values supplied by the insert SQL query are already inserted into the table. If you try to assign new value to the column in AFTER insert trigger code then oracle error would be raised. To alter any values supplied by insert SQL query create BEFORE insert trigger.
Q. Explain use of SYSDATE and USER keywords.
SYSDATE is a pseudo column and refers to the current server system date. USER is a pseudo column and refers to the current user logged onto the oracle session. These values come handy when you want to monitor changes happening to the table.
Q. What is the difference between explicit cursor and implicit cursor?
When a single insert, delete or update statement is executed within PL/SQL program then oracle creates an implicit cursor for the same, executes the statement, and closes the cursor. You can check the result of execution using SQL%ROWCOUNT function.
Explicit cursors are created programmatically. The cursor type variable is declared and associated with SQL query. The program then opens a cursor, fetches column information into variables or record type variable, and closes cursor after all records are fetched. To check whether cursor is open or not use function SQL%ISOPEN and to check whether there are any records to be fetched from the cursor use function SQL%FOUND.
Q. Why does a query in Oracle run faster when ROWID is used as a part of the where clause?
ROWID is the logical address of a row - it is not a physical column. It is composed of file number, data block number and row number within data block. Therefore I/O time is minimized retrieving the row, resulting in a faster query.
Q. What type of exception will be raised in the following situations:
a> select..into statement returns more than one row.
b> select..into statement does not return any row.
c> insert statement inserts a duplicate record.
The errors returned are:
a> TOO_MANY_ROWS
b> NO_DATA_FOUND
c> DUP_VAL_ON_INDEX
Oracle Interview Questions
and Answers : SQL
1. To see current user name
Sql> show user;
2. Change SQL prompt name
SQL> set sqlprompt “Manimara
> “
Manimara >
Manimara >
3. Switch to DOS prompt
SQL> host
4. How do I eliminate the
duplicate rows ?
SQL> delete from table_name
where rowid not in (select max(rowid) from table group by
duplicate_values_field_name);
or
SQL> delete
duplicate_values_field_name dv from table_name ta where rowid <(select
min(rowid) from
table_name tb where
ta.dv=tb.dv);
Example.
Table Emp
Empno Ename
101 Scott
102 Jiyo
103 Millor
104 Jiyo
105 Smith
delete ename from emp a where
rowid < ( select min(rowid) from emp b where a.ename = b.ename);
The output like,
Empno Ename
101 Scott
102 Millor
103 Jiyo
104 Smith
5. How do I display row
number with records?
To achive this use rownum
pseudocolumn with query, like SQL> SQL> select rownum, ename from emp;
Output:
1 Scott
2 Millor
3 Jiyo
4 Smith
6. Display the records
between two range
select rownum, empno, ename
from emp where rowid in
(select rowid from emp where
rownum <=&upto
minus
select rowid from emp where
rownum<&Start);
Enter value for upto: 10
Enter value for Start: 7
ROWNUM EMPNO ENAME
--------- --------- ----------
1 7782 CLARK
2 7788 SCOTT
3 7839 KING
4 7844 TURNER
7. I know the nvl function
only allows the same data type(ie. number or char or date
Nvl(comm, 0)), if commission
is null then the text “Not Applicable” want to display, instead of
blank space. How do I write
the query?
SQL> select
nvl(to_char(comm.),'NA') from emp;
Output :
NVL(TO_CHAR(COMM),'NA')
-----------------------
NA
300
500
NA
1400
NA
NA
8. Oracle cursor : Implicit
& Explicit cursors
Oracle uses work areas called
private SQL areas to create SQL statements.
PL/SQL construct to identify
each and every work are used, is called as Cursor.
For SQL queries returning a
single row, PL/SQL declares all implicit cursors.
For queries that returning more
than one row, the cursor needs to be explicitly declared.
9. Explicit Cursor attributes
There are four cursor
attributes used in Oracle
cursor_name%Found,
cursor_name%NOTFOUND, cursor_name%ROWCOUNT, cursor_name%ISOPEN
10. Implicit Cursor
attributes
Same as explicit cursor but
prefixed by the word SQL
SQL%Found, SQL%NOTFOUND,
SQL%ROWCOUNT, SQL%ISOPEN
Tips : 1. Here SQL%ISOPEN is
false, because oracle automatically closed the implicit cursor after
executing SQL statements.
: 2. All are Boolean
attributes.
11. Find out nth highest
salary from emp table
SELECT DISTINCT (a.sal) FROM
EMP A WHERE &N = (SELECT COUNT (DISTINCT (b.sal)) FROM EMP B
WHERE a.sal<=b.sal);
Enter value for n: 2
SAL
---------
3700
12. To view installed Oracle
version information
SQL> select banner from
v$version;
13. Display the number value
in Words
SQL> select sal,
(to_char(to_date(sal,'j'), 'jsp'))
from emp;
the output like,
SAL
(TO_CHAR(TO_DATE(SAL,'J'),'JSP'))
---------
-----------------------------------------------------
800 eight hundred
1600 one thousand six hundred
1250 one thousand two hundred
fifty
If you want to add some text
like,
Rs. Three Thousand only.
SQL> select sal "Salary
",
(' Rs. '||
(to_char(to_date(sal,'j'), 'Jsp'))|| ' only.'))
"Sal in Words" from
emp
/
Salary Sal in Words
-------
------------------------------------------------------
800 Rs. Eight Hundred only.
1600 Rs. One Thousand Six
Hundred only.
1250 Rs. One Thousand Two
Hundred Fifty only.
14. Display Odd/ Even number
of records
Odd number of records:
select * from emp where
(rowid,1) in (select rowid, mod(rownum,2) from emp);
1
3
5
Even number of records:
select * from emp where
(rowid,0) in (select rowid, mod(rownum,2) from emp)
2
4
6
15. Which date function
returns number value?
months_between
16. Any three PL/SQL
Exceptions?
Too_many_rows, No_Data_Found,
Value_Error, Zero_Error, Others
17. What are PL/SQL Cursor
Exceptions?
Cursor_Already_Open,
Invalid_Cursor
18. Other way to replace
query result null value with a text
SQL> Set NULL ‘N/A’
to reset SQL> Set NULL ‘’
19. What are the more common
pseudo-columns?
SYSDATE, USER , UID, CURVAL,
NEXTVAL, ROWID, ROWNUM
20. What is the output of
SIGN function?
1 for positive value,
0 for Zero,
-1 for Negative value.
21. What is the maximum
number of triggers, can apply to a single table?
12 triggers.
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