用not exists 为什么得不到我的结果。

[yeag]

tableroom表中有tname字段（主键），Orderstable表中也有tname字段（外键）。
tableroom表中有6条记录，TN1，TN2，TN3，TN4，TN5，TN6
Orderstable表中有2条记录，TN1，TN2
用 select Orderstable.Tname from Orderstable 可得到两条记录TN1，TN2
但用 not exists 却得不到我的结果为什么。
语法如下：
select tname from tableroom
where not exists
(select Orderstable.Tname from Orderstable)

结果是空。
真实的结果应该是TN3，TN4，TN5，TN6。

为什么

[victorycyz]

select tname from tableroom
where tname not exists
(select Orderstable.Tname from Orderstable)

[welyngj]

select tname from tableroom a
where not exists
(select Orderstable.Tname from Orderstable b where a.tname=b.tname)

[pengdali]

select tname from tableroom
where tname not in
(select Orderstable.Tname from Orderstable)

[pengdali]

select tname from tableroom a
where not exists
(select 1 from Orderstable where tname=a.tname)

[zjcxc]

如果是要得到楼主要求的结果,应该是用:

select tname from tableroom a
where not exists
(select 1 from Orderstable where tname=a.tname)

[zjcxc]

或者:

select a.tname from tableroom a left join orderstable b on a.tname=b.tname
where b.tname is null

[victorycyz]

or:
select
tname
from
tableroom a left join orderstable b on a.tname=b.tname
where
b.tname is null

[txlicenhe]

select tname from tableroom
where tname not in
(select Orderstable.Tname from Orderstable)

或
select tname from tableroom a
where not exists
(select Orderstable.Tname from Orderstable where Tname = a.tname)

[yeag]

是in效率高还是exists高呢。

[welyngj]

exists的高

[FBugFramework]

exists高