UNION
This is basically the same as in mathematical sets. Consider the following table:
| birtday | first_name | family_name | e_id |
|---|---|---|---|
| 1990-11-10 | Anne | Smith | 1 |
| 2000-02-05 | David | Jones | 2 |
| 2995-05-09 | William | Taylor | 3 |
Executing the following query would give the following table:
SELECT first_name AS name
FROM Employees
UNION
SELECT family_name
FROM Employees;
| name |
|---|
| Anne |
| David |
| William |
| Smith |
| Jones |
| Taylor |
The name of the column comes from the first SELECT statement.
The things in the UNION must have similar data-types and also the same number of columns.
Similar Data-Types
Different implementations have different definitions of what classes as a “similar data-type”. Generally you can mix most things but you should check the docs for your specific implementation.