SQL PART 4 D

Orders:

Prod_ID Product Employee_ID
234 Printer 01
657 Table 03
865 Chair 03

Referring to Two Tables
We can select data from two tables by referring to two tables, like this:
Example
Who has ordered a product, and what did they order?

SELECT Employees.Name, Orders.Product FROM Employees, Orders
WHERE Employees.Employee_ID=Orders.Employee_ID
Result
Name Product
Hansen, Ola Printer
Svendson, Stephen Table
Svendson, Stephen Chair
Example
Who ordered a printer?
SELECT Employees.Name FROM Employees, Orders WHERE Employees.Employee_ID=Orders.Employee_ID AND Orders.Product='Printer'

Result
Name
Hansen, Ola
Using Joins
OR we can select data from two tables with the JOIN keyword, like this:
Example INNER JOIN
Syntax

SELECT field1, field2, field3 FROM first_table INNER JOIN second_table
ON first_table.keyfield = second_table.foreign_keyfield

Who has ordered a product, and what did they order?

SELECT Employees.Name, Orders.Product FROM Employees

INNER JOIN Orders

ON Employees.Employee_ID=Orders.Employee_ID

The INNER JOIN returns all rows from both tables where there is a match. If there are rows in Employees that do not have matches in Orders, those rows will not be listed.

Result
Name Product
Hansen, Ola Printer
Svendson, Stephen Table
Svendson, Stephen Chair

Example LEFT JOIN
Syntax

SELECT field1, field2, field3 FROM first_table LEFT JOIN second_table
ON first_table.keyfield = second_table.foreign_keyfield

List all employees, and their orders - if any.

SELECT Employees.Name, Orders.Product FROM Employees

LEFT JOIN Orders

ON Employees.Employee_ID=Orders.Employee_ID

The LEFT JOIN returns all the rows from the first table (Employees), even if there are no matches in the second table (Orders). If there are rows in Employees that do not have matches in Orders, those rows also will be listed.

Result

Name Product

Hansen, Ola Printer
Svendson, Tove
Svendson, Stephen Table
Svendson, Stephen Chair
Pettersen, Kari

Example RIGHT JOIN
Syntax

SELECT field1, field2, field3 FROM first_table RIGHT JOIN second_table

ON first_table.keyfield = second_table.foreign_keyfield

List all orders, and who has ordered - if any.

SELECT Employees.Name, Orders.Product FROM Employees RIGHT JOIN Orders
ON Employees.Employee_ID=Orders.Employee_ID

The RIGHT JOIN returns all the rows from the second table (Orders), even if there are no matches in the first table (Employees). If there had been any rows in Orders that did not have matches in Employees, those rows also would have been listed.

Result

Name Product
Hansen, Ola Printer
Svendson, Stephen Table
Svendson, Stephen Chair

Example

SELECT Employees.Name FROM Employees INNER JOIN Orders

ON Employees.Employee_ID=Orders.Employee_ID
WHERE Orders.Product = 'Printer'

Result

Name
Hansen, Ola

SQL UNION and UNION ALL

UNION

The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type.

Note: With UNION, only distinct values are selected.

SQL Statement 1

UNION

SQL Statement 2

Employees_Norway:
Employee_ID E_Name
01 Hansen, Ola
02 Svendson, Tove
03 Svendson, Stephen
04 Pettersen, Kari
Employees_USA:
Employee_ID E_Name
01 Turner, Sally
02 Kent, Clark
03 Svendson, Stephen
04 Scott, Stephen

Using the UNION Command
Example
List all different employee names in Norway and USA:
SELECT E_Name FROM Employees_Norway
UNION
SELECT E_Name FROM Employees_USA

Result
Name
Hansen, Ola
Svendson, Tove
Svendson, Stephen
Pettersen, Kari
Turner, Sally
Kent, Clark
Scott, Stephen

Note: This command cannot be used to list all employees in Norway and USA. In the example above we have two employees with equal names, and only one of them is listed. The UNION command only selects distinct values.

UNION ALL

The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.

SQL Statement 1
UNION ALL
SQL Statement 2

Using the UNION ALL Command

Example
List all employees in Norway and USA:

SELECT E_Name FROM Employees_Norway
UNION ALL
SELECT E_Name FROM Employees_USA

Result
Name
Hansen, Ola
Svendson, Tove
Svendson, Stephen
Pettersen, Kari
Turner, Sally
Kent, Clark
Svendson, Stephen
Scott, Stephen