The Oracle UNION operator is used to combine the result sets of 2 or more Oracle SELECT statements. It removes duplicate rows between the various SELECT statements.
Each SELECT statement within the UNION operator must have the same number of fields in the result sets with similar data types.
Syntax
The syntax for the UNION operator in Oracle/PLSQL is:
SELECT expression1, expression2, ... expression_n FROM tables [WHERE conditions] UNION SELECT expression1, expression2, ... expression_n FROM tables [WHERE conditions];
Parameters or Arguments
expression1, expression2, expression_n
The columns or calculations that you wish to retrieve.
tables:
The tables that you wish to retrieve records from. There must be at least one table listed in the FROM clause.
WHERE conditions
Optional. The conditions that must be met for the records to be selected.
Note
There must be same number of expressions in both SELECT statements.