Question

I have Parent + Reference tables where
Reference table is as follows

Ref_ID    PARENT_ID    
-------------------
1           1            
2           1            
1           2       
3           2       
1           3       
3           3       
4           3       
2           4       
3           4       

Trying to return all distinct parent rows WHERE ref_id in ( 2, 3 )
using a join instead of using a subquery
but duplicates are being returned for parent via join query

Any help is appreciated

FYI – there are 4-7 tables in the query ( depending on user selections )so performance is a huge factor

Answer 1

Ok,

First the code

DECLARE @Temp TABLE(
Ref_ID int,
ParentID int
)

INSERT INTO @Temp VALUES (1 , 1)
INSERT INTO @Temp VALUES (2 , 1)
INSERT INTO @Temp VALUES (1 , 2)
INSERT INTO @Temp VALUES (3 , 2)
INSERT INTO @Temp VALUES (1 , 3)
INSERT INTO @Temp VALUES (3 , 3)
INSERT INTO @Temp VALUES (4 ,3)
INSERT INTO @Temp VALUES (2 , 4)
INSERT INTO @Temp VALUES (3 , 4)

--First Example witout Ref_ID
SELECT 
  DISTINCT ParentID
FROM @Temp
WHERE Ref_ID IN (2,3)

--SECOND Example With Ref_ID
SELECT 
   DISTINCT ParentID,
   Ref_ID
FROM @Temp
WHERE Ref_ID IN (2,3)

The first query returns a distinct record of ParentID’s. It works fine because i haven’t requested for Ref_Id as well.

Just to explain you when you requesting for Ref_ID as well, the query tries to return a distinct ParentID’s. But look at your records you have two ParentID’s which equals to 4
But with Different Ref_ID so he returns them both. both of them matching the query rules