Add support for EXCEPT clause
The EXCEPT clause has this general form:
select_statement EXCEPT select_statement
select_statement is any SELECT statement without an ORDER BY, LIMIT, or FOR UPDATE clause.
The EXCEPT operator computes the set of rows that are in the result of the left SELECT statement but not in the result of the right one.
The result of EXCEPT does not contain any duplicate rows unless the ALL option is specified. With ALL, a row that has m duplicates in the left table and n duplicates in the right table will appear max(m-n,0) times in the result set.