text: -- join.test
  -- 
  -- execsql {
  --       SELECT * FROM t12 NATURAL LEFT JOIN t13
  --       EXCEPT
  --       SELECT * FROM t12 NATURAL LEFT JOIN (SELECT * FROM t13 WHERE b>0);
  -- }
  SELECT * FROM t12 NATURAL LEFT JOIN t13
  EXCEPT
  SELECT * FROM t12 NATURAL LEFT JOIN (SELECT * FROM t13 WHERE b>0);