The SQL UNION operator combines two or more SELECT statements. The SQL UNION Operator The UNION operator is used to combine the result-set...
The SQL UNION operator combines two or more SELECT statements.
The SQL UNION Operator
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.
SQL 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.
SQL UNION ALL Syntax
SELECT column_name(s) FROM table_name1 UNION ALL SELECT column_name(s) FROM table_name2 |
PS: 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.
SQL UNION Example
Look at the following tables:"Employees_Norway":
| E_ID | E_Name |
|---|---|
| 01 | Hansen, Ola |
| 02 | Svendson, Tove |
| 03 | Svendson, Stephen |
| 04 | Pettersen, Kari |
"Employees_USA":
| E_ID | E_Name |
|---|---|
| 01 | Turner, Sally |
| 02 | Kent, Clark |
| 03 | Svendson, Stephen |
| 04 | Scott, Stephen |
Now we want to list all the different employees in Norway and USA.
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 |
|---|
| 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 will be listed. The UNION command selects only distinct values.
SQL UNION ALL Example
Now we want to list all employees in Norway and USA:SELECT E_Name FROM Employees_Norway UNION ALL SELECT E_Name FROM Employees_USA |
Result
| E_Name |
|---|
| Hansen, Ola |
| Svendson, Tove |
| Svendson, Stephen |
| Pettersen, Kari |
| Turner, Sally |
| Kent, Clark |
| Svendson, Stephen |
| Scott, Stephen |
