Thread: select on many-to-many relationship

select on many-to-many relationship

From
ssylla
Date:
Dear list,

assuming I have the following n:n relationship:

t1:
id_project
1
2

t2:
id_product
1
2

intermediary table:
t3
id_project|id_product
1|1
1|2
2|1

How can I create an output like this:
id_project|id_product1|id_product2
1|1|2
2|1|NULL 



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Re: select on many-to-many relationship

From
Виктор Егоров
Date:
2012/11/27 ssylla <stefansylla@gmx.de>:
> assuming I have the following n:n relationship:
>
> intermediary table:
> t3
> id_project|id_product
> 1|1
> 1|2
> 2|1
>
> How can I create an output like this:
> id_project|id_product1|id_product2
> 1|1|2
> 2|1|NULL

I'd said the sample is too simplified — not clear which id_product
should be picked if there're more then 2 exists.
I assumed the ones with smallest IDs.

-- this is just a sample source generator
WITH t3(id_project, id_product) AS (VALUES (1,1),(1,2),(2,1))
-- this is the query
SELECT l.id_project, min(l.id_product) id_product1, min(r.id_product)
id_product2 FROM t3 l LEFT JOIN t3 r ON l.id_project=r.id_project AND l.id_product < r.id_productGROUP BY l.id_project;


--
Victor Y. Yegorov



Re: select on many-to-many relationship

From
Sergey Konoplev
Date:
On Tue, Nov 27, 2012 at 2:13 AM, ssylla <stefansylla@gmx.de> wrote:
> id_project|id_product
> 1|1
> 1|2
> 2|1
>
> How can I create an output like this:
> id_project|id_product1|id_product2
> 1|1|2
> 2|1|NULL

You can use the crostab() function from the tablefunc module
(http://www.postgresql.org/docs/9.2/static/tablefunc.html). It does
exactly what you need.

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