例如，表table_a

表table_b

现在需要找出A表中id不存在于B表的记录

SELECT
  table_a.*
FROM
  table_a
LEFT JOIN table_b ON (table_a.id = table_b.id)
WHERE
  table_b.id IS NULL;

如果需要删除A表中这些记录

DELETE table_a
FROM
  table_a
LEFT JOIN table_b ON (table_a.id = table_b.id)
WHERE
  table_b.id IS NULL;