无法准备语句（1 次滥用聚合函数 COUNT()）

Question

我正在尝试运行以下查询：

select U.userEmail, U.firstname + ' ' + U.surName as fullname 
from tblUser as U inner join tblHitchhhiker as H on U.userEmail= H.userEmail
left outer join tblGetAsk as G  on H.userEmail = G.userEmail
where COUNT(G.userEmail) = 0

但我收到以下错误：

错误 1：无法准备语句（1 滥用聚合函数 计数（））

我真正想做的是：只选择也是搭便车者并且在 tblGetAsk 中有 0 条记录的用户。

Answer 1

...在 tblGetAsk 中有 0 条记录

这意味着tblGetAsk 中没有与tblUser 的userEmail 匹配的行，因此您的条件应该是where G.userEmail IS NULL：

select U.userEmail, U.firstname + ' ' + U.surName as fullname 
from tblUser as U 
inner join tblHitchhhiker as H on U.userEmail= H.userEmail
left outer join tblGetAsk as G on U.userEmail = G.userEmail
where G.userEmail IS NULL

您也可以使用EXISTS 和NOT EXISTS 代替连接：

select U.userEmail, U.firstname + ' ' + U.surName as fullname 
from tblUser as U 
where exists (select 1 from tblHitchhhiker as H where H.userEmail= U.userEmail)
  and not exists (select 1 from tblGetAsk as G where G.userEmail = U.userEmail)