Executes one or more SQL statements in an IF . . . END IF block
    only when the value of an IF predicate evaluates to TRUE. A
    predicate, also called a conditional expression, specifies a
    condition that SQL evaluates to TRUE, FALSE, or UNKNOWN. If
    the predicate evaluates to TRUE, SQL executes the statement in
    the THEN clause. If the predicate does not evaluate to TRUE,
    SQL evaluates the predicate in any ELSEIF clauses. If the IF
    statement contains no ELSEIF clauses, SQL executes any statements
    in the ELSE clause.