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

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

人气：385 ℃　时间：2020-02-06 06:06:55

解答

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表数据子查询只是把计数传过去啊主查询如何得知具体数据行？nono, 这么看：select * from B where ?= 0where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=0select count(1) as num from A where A.ID = B.ID不等于0啊