MySQL - Union
The UNION operator is used to combine the result-set of two or more SELECT statements. Notice that each SELECT statement within the UNION must have the same number of columns. The columns must also have similar data types. Also, the columns in each SELECT statement must be in the same order.
MySQL UNION Syntax
SELECT column_name(s) FROM table_name1
UNION
SELECT column_name(s) FROM table_name2
Note: The UNION operator selects only distinct values by default. To allow duplicate values, use UNION ALL.
MySQL UNION ALL Syntax
SELECT column_name(s) FROM table_name1
UNION ALL
SELECT column_name(s) FROM table_name2
The column names in the result-set of a UNION are always equal to the column names in the first SELECT statement in the UNION.
Look at the following tables:
"Employees_Bali":
| E_ID | E_Name |
|---|
| 01 | Putu Sanjaya |
| 02 | Made Sanjaya |
| 03 | Haru Sanjaya |
| 04 | Ketut Sanjaya |
"Employees_Lombok":
| E_ID | E_Name |
|---|
| 01 | Lalu Selamet |
| 02 | Mamiq Djumenah |
| 03 | Haru Sanjaya |
| 04 | Amaq Sadikin |
Now we want to list all the different employees in Bali and Lombok.
We use the following SELECT statement:
SELECT E_Name FROM Employees_Norway
UNION
SELECT E_Name FROM Employees_USA
The result-set will look like this:
E_Name
Putu Sanjaya
Made Sanjaya
Haru Sanjaya
Ketut Sanjaya
Lalu Selamet
Mamiq Djumenah
Amaq Sadikin
Note: This command cannot be used to list all employees in Bali and Lombok. In the example above we have two employees with equal names, and only one of them will be listed. The UNION command selects only distinct values.
Now we want to list all employees in Bali and Lombok:
SELECT E_Name FROM Employees_Norway
UNION ALL
SELECT E_Name FROM Employees_USA
Result
E_Name
Putu Sanjaya
Made Sanjaya
Haru Sanjaya
Ketut Sanjaya
Lalu Selamet
Mamiq Djumenah
Haru Sanjaya
Amaq Sadikin
