Question

你可以运行它并告诉我为什么结果集只有两行。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，因此您只需将其粘贴到查询工具

即可

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来自#appSteps的所有3个非空行都没有返回

Answer 1

原因是您在WHERE子句中包含了右侧表格。您应该将其移至ON的{{1}}条件：

LEFT JOIN

这样做的原因是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子句在 WHERE之后评估，然后从{{1}过滤出LEFT JOIN个结果}}

在NULL子句中包含LEFT JOIN的右侧表格（或LEFT JOIN的左侧表格）有效地将RIGHT JOIN转换为{ {1}}。

为什么我的t-sql离开加入不起作用？

1 个答案: