显示没有匹配项的结果 - SQL Server 2008

Question

我有以下查询显示两个表之间匹配的记录：

select DISTINCT ug.name, ug.payroll_no, esr.assignment from user_group AS ug
inner join esrtraining AS esr on ug.payroll_no = SUBSTRING(esr.assignment, 2, 8)

我怎么能这样做呢？例如显示“User_Group”中没有匹配的名称？感谢。

Answer 1

您可以使用NOT EXISTS以相同条件执行此操作：

select DISTINCT ug.name, ug.payroll_no
from user_group AS ug
Where Not Exists
(
    Select  *
    From    esrtraining AS esr 
    Where   ug.payroll_no = SUBSTRING(esr.assignment, 2, 8)
)   

Answer 2

使用OUTER连接而不是INNER，然后在第二个表中不可为空的列的WHERE子句中查找NULL：

select DISTINCT ug.name, ug.payroll_no, esr.assignment 
from user_group AS ug
left join esrtraining AS esr on ug.payroll_no = SUBSTRING(esr.assignment, 2, 8)
where esr.assignment is null

这称为排除加入。