sql - SQL，我如何转换为逻辑（'不在'到'不存在'）

Question

Delete From StudentTb Where StudentType in (1, 2) and StudentI not in
(
    Select lg.StudentI From StudentLog lg
)

我试图将上面的代码更改为以下...但是，我不确定它是否正确。

Delete From StudentTb Where StudentType in (1, 2) 
    and Not Exists 
    (
        Select 'x' 
        From StudentLog lg                          
        Where StudentI= lg.StudentI
    )

谁能帮帮我吗？

Answer 1

Delete From StudentTb  s
Where StudentType in (1, 2) 
and Not Exists 
(
    Select *
    From StudentLog                          
    Where StudentI = s.StudentI
)

在子查询中，对列的非限定引用（不指定表名或别名）首先被假定在子查询本身引用的表中，而不是在外部查询的其他表中。因此，在您的 sql 语法中，谓词的两侧是子查询表Where StudentI = lg.StudentI中的同一列StudentLog。

Answer 2

您做错的只是没有在子查询中指定正确的别名：

Delete From StudentTb Where StudentType in (1, 2) 
    and Not Exists 
    (
        Select 'x' 
        From StudentLog lg                          
        Where StudentTb.StudentI= lg.StudentI
    )

注意 - select 语句中的列不会影响最终结果，但仍然首选 write*而不是'x'

Answer 3

not in 和 not exist 并不总是具有相同的含义。我假设您想要转换，因为“不在”往往很慢。这是逻辑始终匹配的另一种方式。

Delete From StudentTb 
Where StudentType in (1, 2) 
and StudentI in
(
select StudentI
from StudentTb
except 
Select StudentI From StudentLog    
)