1、差集( except )
select a from t_a
except
select a from t_b
-- 也可写作：
select a from t_a where a not in (select a from t_b)
-- 多个字段时：
select a,b from t_a
except
select a,b from t_b
-- 多字段的查集也可写成：
select a,b from t_a where (a,b) not in (select a,b from t_b)
2、交集( intersect )
select a from t_a
intersect
select a from t_b
-- 也可写作:
select a from t_a where a in (select a from t_b)
3、并集( union )
select a from t_a
union distinct
select a from t_b