SQL集合運算：差集、交集、並集

一、差集( except )

select a from t_a

except

select a from t_b

-- 也可寫做：

select a from t_a where a not in (select a from t_b)

-- 多個字段時：

select a,b from t_a

except

select a,b from t_b

-- 多字段的查集也可寫成：

select a,b from t_a where (a,b) not in (select a,b from t_b)

二、交集( intersect )

select a from t_a

intersect

select a from t_b

-- 也可寫做:

select a from t_a where a in (select a from t_b)

三、並集( union )

select a from t_a

union distinct

select a from t_b