为什么我的 t-sql 左连接不起作用？

你能运行这个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

我不明白为什么 #appSteps 中的所有 3 个非空行都没有返回

原因是因为您将右侧表格包含在WHERE条款。您应该将其移至ON的条件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

它这样做的原因是因为WHERE条款被评估after the LEFT JOIN，然后过滤掉你的NULL结果来自LEFT JOIN.

包括右边的表LEFT JOIN（或者左边的表RIGHT JOIN） 在里面WHERE条款有效地改变了OUTER JOIN进入一个INNER JOIN.