The syntax of the UNION clause is as follows:
select_statement UNION [ ALL ] select_statement
select_statement is a
SELECT statement that does not contain an
ORDER BY or
FOR UPDATE clause. You can enclose the
ORDER BY clause in parentheses to attach it to a sub-expression. Without parentheses, these
clauses are applied to the result of the
UNION clause, not to the expression on the right side.
UNION operator computes the set union of the rows returned by the involved
SELECT statements. If a row is included in at least one of two result sets, the row is in
the set union of the two result sets. The two
SELECT statements that represent the direct operands of the
UNION clause must produce the same number of columns. The corresponding columns must be
of compatible data types.
The result of the
UNION clause contains duplicate rows only if the
ALL option is specified. The
ALL option prevents elimination of duplicate rows.
Unless otherwise specified in parentheses, multiple
UNION operators in the same
SELECT statement are evaluated from left to right.
FOR UPDATE clause may not be specified either for a
UNION result or for an input of a