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SQL QuizSQL UNION Operator
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
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
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:
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:
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 |
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