如何简化多个“不在”查询？使用多个子查询是否有效：不在(…)和不在(..)和不在(…)
我正在使用计数(抱歉忘记了)
Select count (VisitorID) from Company where VisitorID not in (select VisitorID from UserLog where ActionID = 2 ) and VisitorID not in (select VisitorID from Supplies where productID = 4)
解决方法
Select count (VisitorID) from Company C where NOT EXISTS (select * from UserLog U where ActionID = 2 AND C.VisitorID = U.VisitorID) AND NOT EXISTS (select * from Supplies S where productID = 4 AND S.VisitorID = U.VisitorID)
为什么不存在？
> NOT IN：UserLog或Supplies中的任何NULL VisitorID值表示不匹配
>(LEFT JOIN)：如果每个VisitorID有多个UserLog或Supplies,则为多个输出行.需要DISTINCT改变计划
通常,NOT EXISTS是唯一正确的选项
