Question

I have the following query:

select * from ACADEMIC a
left join RESIDENCY r on a.PEOPLE_CODE_ID = r.PEOPLE_CODE_ID
where a.ACADEMIC_TERM='Fall' 
and r.ACADEMIC_TERM='Fall'
and a.ACADEMIC_SESSION=''
and a.ACADEMIC_YEAR = (Select Year(GetDate())) 
and r.ACADEMIC_YEAR = (Select Year(GetDate()))
and (CLASS_LEVEL LIKE 'FR%'
     OR a.CLASS_LEVEL LIKE 'SO'
     OR a.CLASS_LEVEL LIKE 'JR'
     OR a.CLASS_LEVEL LIKE 'SR%') 
and r.RESIDENT_COMMUTER='R'

For each person in the database it returns two rows with identical information. Yet, when I do the same query without the left join:

select * from ACADEMIC a
where a.ACADEMIC_TERM='Fall' 
and a.ACADEMIC_SESSION=''
and a.ACADEMIC_YEAR = (Select Year(GetDate())) 
and (CLASS_LEVEL LIKE 'FR%'
     OR a.CLASS_LEVEL LIKE 'SO'
     OR a.CLASS_LEVEL LIKE 'JR'
     OR a.CLASS_LEVEL LIKE 'SR%') 
     ORDER BY PEOPLE_ID

It returns only one row for each person. I’m doing a left join – why is it adding an extra row? Shouldn’t it only do that if I add a right join?

(Updated formatting for consistency/readability.)

Answer 1

I’m doing a left join – why is it adding an extra row?

You are doing an INNER JOIN.

Conditions like this:

and r.ACADEMIC_TERM='Fall'

effectively filter out the fake rows produced by the left join, leaving only the records that would be returned by an INNER JOIN on the same conditions.

The reason because you are getting two records per academic is that there are two records in residency per academic that satisfy the other join condition.

If you want to return only one residency per academic, you need to define which one would it be.