A table “union” means that the rows of one table are combined under the rows of another table. Unions are rarely used.

Table A:

Table B:

Result table of a union:

The definition (data type) and number of columns in the first table must exactly match the definition and number of columns in the second table. Since the source tables may have different column names, the result table has no column names.

A table “join” means that data from two or more tables can be used to create a single result table. The join makes an RDBMS a very powerful tool.

EMPLOYEE_TBL:

PAYROLL_TBL:

Joined Result Table:

Table “selection” means that all or some of the table rows can be retrieved into the result table.

Selected Result Table: