This topic describes the UNION clause.
The following statement provides the syntax of the UNION clause:
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 the clause to a sub-expression. If parentheses are
not used, these clauses are applied to the result of the
UNION clause instead of the expression on the right side.
UNION operator computes the set union of the rows that are 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 generate 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 duplicate rows from being eliminated.
Unless otherwise specified in parentheses, multiple
UNION operators in the same
SELECT statement are evaluated from left to right.
FORUPDATE clause may not be specified for a
UNION result or for an input of a