SQL語句理解 select * from B where (select count(1) as num from A where A.ID = B.ID) = 0

SQL語句理解 select * from B where (select count(1) as num from A where A.ID = B.ID) = 0
select * from B where (select count(1) as num from A where A.ID = B.ID) = 0
請問如何理解,為什么可以在B表中排除A表的數(shù)據(jù)

其他人氣：159 ℃時(shí)間：2020-02-06 06:06:55

優(yōu)質(zhì)解答

子查詢中select count(1) as num from A where A.ID = B.ID
就是統(tǒng)計(jì)B表和A表用ID連接的行數(shù)
子查詢的行數(shù)=0,就可以實(shí)現(xiàn)從B表中排除A表數(shù)據(jù)子查詢只是把計(jì)數(shù)傳過去啊主查詢?nèi)绾蔚弥唧w數(shù)據(jù)行？nono, 這么看：select * from B where ?= 0where中條件其實(shí)是，什么等于0?代入(select count(1) as num from A where A.ID = B.ID)就是A和B通過ID連接后,COUNT(*)計(jì)數(shù)=0select count(1) as num from A where A.ID = B.ID不等于0啊

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SQL語句理解 select * from B where (select count(1) as num from A where A.ID = B.ID) = 0

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