Remove Duplicates from LEFT OUTER JOIN

Question

My question is quite similar to Restricting a LEFT JOIN , with a variation.

Assuming I have a table SHOP and another table LOCATION. Location is a sort of child table of table SHOP, that has two columns of interest, one is a Division Key (calling it just KEY) and a "SHOP" number. This matches to the Number "NO" in table SHOP.

I tried this left outer join:

SELECT S.NO, L.KEY
FROM SHOP S
LEFT OUTER JOIN LOCATN L ON S.NO = L.SHOP

but I'm getting a lot of duplicates since there are many locations that belong to a single shop. I want to eliminate them and just get a list of "shop, key" entries without duplicates.

The data is correct but duplicates appear as follows:

SHOP     KEY
 1       XXX
 1       XXX
 2       YYY
 3       ZZZ
 3       ZZZ  etc.

I would like the data to appear like this instead:

SHOP     KEY
 1       XXX
 2       YYY
 3       ZZZ  etc.

SHOP table:

 NO
 1       
 2       
 3       

LOCATION table:

 LOCATION   SHOP  KEY
   L-1       1    XXX   
   L-2       1    XXX   
   L-3       2    YYY   
   L-4       3    YYY   
   L-5       3    YYY   

(ORACLE 10g Database)

Answer 1

You need to GROUP BY 'S.No' & 'L.KEY'

SELECT S.NO, L.KEY 
FROM SHOP S 
LEFT OUTER JOIN LOCATN L 
ON S.NO = L.SHOP
GROUP BY S.NO, L.KEY

Answer 2

EDIT Following the update in your scenario

I think you should be able to do this with a simple sub query (though I haven't tested this against an Oracle database). Something like the following

UPDATE shop s
SET divnkey = (SELECT DISTINCT L.KEY FROM LOCATN L WHERE S.NO = L.SHOP)

The above will raise an error in the event of a shop being associated with locations that are in multiple divisions.

If you just want to ignore this possibility and select an arbitrary one in that event you could use

UPDATE shop s
SET divnkey = (SELECT MAX(L.KEY) FROM LOCATN L WHERE S.NO = L.SHOP)

Answer 3

I had this problem too but I couldn't use GROUP BY to fix it because I was also returning TEXT type fields. (Same goes for using DISTINCT).

This code gave me duplicates:

select mx.*, case isnull(ty.ty_id,0) when 0 then 'N' else 'Y' end as inuse 
from master_x mx 
left outer join thing_y ty on mx.rpt_id = ty.rpt_id

I fixed it by rewriting it thusly:

select mx.*, 
case when exists (select 1 from thing_y ty where mx.rpt_id = ty.rpt_id) then 'Y' else 'N' end as inuse
from master_x mx 

As you can see I didn't care about the data within the 2nd table ( thing_y ), just whether there was greater than zero matches on the rpt_id within it. (FYI: rpt_id was also not the primary key on the 1st table, master_x ).