Why is my t-sql left join not working?

Question

Can you run this and tell me why the result set only has two rows. It should have three and look like this...

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

Here is the sql so you can just paste it into your query tool

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

I cannot figure out why all 3 non null rows from #appSteps are not coming back

Answer 1

The reason is because you are including the right-hand table in the WHERE clause. You should move that to the ON condition of the 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

The reason it does this is because the WHERE clause is evaluated after the LEFT JOIN, which then filters out your NULL results from the LEFT JOIN.

Including the right-hand table of a LEFT JOIN (or the left-hand table of a RIGHT JOIN) in the WHERE clause effectively transforms the OUTER JOIN into an INNER JOIN.