1、差集( except )select a from t_aexcept
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_aexcept
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_aintersectselect 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_aunion distinctselect a from t_b
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