为什么我的T-SQL左联接不起作用？

Question

您可以运行此命令并告诉我为什么结果集只有两行吗？它应该有三个，看起来像这样…

appId    stepId       section       start
101      1           Section 1     2016-01-03 00:00:00.000
101      2           Section 2     2016-01-03 00:00:00.000
101      10          Section 3     NULL

这是sql，因此您可以将其粘贴到查询工具中

create table #appSteps(stepId decimal, section nvarchar(50))
insert into #appSteps (stepId, section) values (1, 'Section 1')
insert into #appSteps (stepId, section) values (2, 'Section 2')
insert into #appSteps (stepId, section) values (3, null)
insert into #appSteps (stepId, section) values (4, null)
insert into #appSteps (stepId, section) values (10, 'Section 3')

create table #appProgress(stepId decimal, appId int, start datetime)
insert into #appProgress (stepId, appId, start) values (1, 101, '1/3/2016')
insert into #appProgress (stepId, appId, start) values (2, 101, '1/3/2016')
insert into #appProgress (stepId, appId, start) values (3, 101, '1/3/2016')
insert into #appProgress (stepId, appId, start) values (4, 101, '1/3/2016')


select p.appId, s.stepId, s.section, p.start
from #appSteps s with (nolock)
left join #appProgress p on s.stepId = p.stepId
where s.section is not null
and p.appId = 101

drop table #appSteps
drop table #appProgress

我无法弄清楚为什么#appSteps中的所有3个非空行都没有返回

Answer 1

原因是因为您在WHERE子句中包含了右侧表。您应该将其移至以下ON条件LEFT JOIN：

Select    P.appId, S.stepId, S.section, P.start
From      #appSteps    S   With (NoLock)
Left Join #appProgress P   On  S.stepId = P.stepId 
                           And P.appId = 101
Where     S.section Is Not Null

这样做的原因是，该WHERE子句是 在后面 进行评估的LEFT JOIN，然后从中过滤出您的NULL结果LEFT JOIN。

在子句中包含的右手表LEFT JOIN（或的左手表RIGHT JOIN）可将WHERE转换OUTER JOIN为INNER JOIN。