Thread: limit left join rows to 1

Hi.

Is it possible to limit number of left join rows that match condition to
1? I don't want to have hits from first table multiplied when more then
1 row matches on left join condition.

Thank you
Jure

Jure Ložar <jure.lozar@madalbal.si> schrieb:

> Hi.
>
> Is it possible to limit number of left join rows that match condition to 1?
> I don't want to have hits from first table multiplied when more then 1 row
> matches on left join condition.

I'm not sure if i understand you correctly, but perhaps this is what you
are searching for:

Suppose, you have 2 tables, master and detail:

test=# select * from master;
 id
----
  1
  2
(2 rows)

test=# select * from detail;
 id | val
----+-----
  1 | 200
  2 | 200
  1 | 100
(3 rows)


This is the left join:

test=# select m.id, d.val from master m left join detail d on m.id=d.id;
 id | val
----+-----
  1 | 100
  1 | 200
  2 | 200
(3 rows)


But you need only one row from detail, which? Suppose, this one with the
max(val) value:

test=# select m.id, d.val from master m left join (select id, max(val)
as val from detail group by id) d on m.id=d.id;
 id | val
----+-----
  1 | 200
  2 | 200
(2 rows)


Is this okay for you?


Andreas
--
Really, I'm not out to destroy Microsoft. That will just be a completely
unintentional side effect.                              (Linus Torvalds)
"If I was god, I would recompile penguin with --enable-fly."    (unknow)
Kaufbach, Saxony, Germany, Europe.              N 51.05082°, E 13.56889°

Andreas Kretschmer wrote:

>Jure Ložar <jure.lozar@madalbal.si> schrieb:
>
>
>
>>Hi.
>>
>>Is it possible to limit number of left join rows that match condition to 1?
>>I don't want to have hits from first table multiplied when more then 1 row
>>matches on left join condition.
>>
>>
>
>I'm not sure if i understand you correctly, but perhaps this is what you
>are searching for:
>
>Suppose, you have 2 tables, master and detail:
>
>test=# select * from master;
> id
>----
>  1
>  2
>(2 rows)
>
>test=# select * from detail;
> id | val
>----+-----
>  1 | 200
>  2 | 200
>  1 | 100
>(3 rows)
>
>
>This is the left join:
>
>test=# select m.id, d.val from master m left join detail d on m.id=d.id;
> id | val
>----+-----
>  1 | 100
>  1 | 200
>  2 | 200
>(3 rows)
>
>
>But you need only one row from detail, which? Suppose, this one with the
>max(val) value:
>
>test=# select m.id, d.val from master m left join (select id, max(val)
>as val from detail group by id) d on m.id=d.id;
> id | val
>----+-----
>  1 | 200
>  2 | 200
>(2 rows)
>
>
>Is this okay for you?
>
>
>Andreas
>
>
Yes. It's good. Not exactly what I ment but it works.

Thank you.

Jure