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1.

select * from  B  where (select count(1) as num from A where A.id = B.uid) = 0  //查询速度最快

2.

select A.id from A left join B on A.id=B.uid where B.uid is null //左连接查找，查询速度稍慢与上一条

3.

select distinct A.id from  A where A.id not in (select uid from B) //not in 简单粗暴，但是属于慢查询，数据量很大不建议使用

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