SQL语句理解 select * from B where (select count(1) as num from A

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SQL语句理解 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
请问如何理解,为什么可以在B表中排除A表的数据

子查询中select count(1) as num from A where A.ID = B.ID
就是统计B表和A表用ID连接的行数
子查询的行数=0,就可以实现从B表中排除A表数据
再问：子查询只是把计数传过去啊主查询如何得知具体数据行？
再答： nono, 这么看： select * from B where ?= 0 where中条件其实是，什么等于0? 代入 (select count(1) as num from A where A.ID = B.ID) 就是A和B通过ID连接后,COUNT(*)计数=0
再问： select count(1) as num from A where A.ID = B.ID 不等于0啊

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