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Re: query two tables using same lookup table

From: David Johnston <polobo(at)yahoo(dot)com>
To: ssylla <stefansylla(at)gmx(dot)de>
Cc: "pgsql-sql(at)postgresql(dot)org" <pgsql-sql(at)postgresql(dot)org>
Subject: Re: query two tables using same lookup table
Date: 2012-07-23 04:45:47
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-sql
On Jul 22, 2012, at 23:04, ssylla <stefansylla(at)gmx(dot)de> wrote:

> Dear list, 
> assuming I have two tables as follows 
> t1: 
> id_project|id_auth 
> 1|1 
> 2|2 
> t2: 
> id_project|id_auth 
> 1|2 
> 2|1 
> and a lookup-table: 
> t3 
> id_auth|name_auth 
> 1|name1 
> 2|name2 
> Now I want to query t1 an t2 using the 'name_auth' column of lookup-table
> t3, so that I get the following output: 
> id_project|name_auth_t1|name_auth_t2 
> 1|name1|name2 
> 2|name2|name1 
> Any ideas? 
> Thanks- 
> Stefan

Not tested, may need minor syntax cleanup but the theory is sound.

With pj as (
Select id_project, id_name1, id_name2
From (select id_project, id_auth as id_auth1 from t1) s1
Natural Full outer join
(select id_project, id_auth as id_auth2 from t2) s2
Select pj.id_project, n1.name_auth, n2.name_auth
From pj
Left join t3 as n1 on (id_auth1 = id_auth)
Left join t3 as n2 on (id_auth2 = id_auth)

Full join the two project tables and give aliases to the duplicate id_auth field.  Then left join against t3 twice (once for eachid_auth) using yet a another set of aliases to distinguish them.

David J.

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