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Re: Select only active entries

From: "A(dot) Kretschmer" <andreas(dot)kretschmer(at)schollglas(dot)com>
To: pgsql-novice(at)postgresql(dot)org
Subject: Re: Select only active entries
Date: 2010-07-21 06:59:48
Message-ID: 20100721065948.GA20566@a-kretschmer.de (view raw or flat)
Thread:
Lists: pgsql-novice
In response to Carel Combrink :
> 
> They are not time stamped but in sequence. The latest active one is  
> basically if  you look at number 5. It goes from active to inactive to  
> active again at time of the query. I want to know if the last entry of  
> ID 5 was active or inactive. And so-forth for all the rest of the  
> ID's. So only select the IDs that were active on their last entry into  
> the database.
> 
> Is there a way of querying the data to obtain only the last entry in  
> the table for a given ID?

There are no UPDATE or DELETE, just only INSERTs into the table?

If yes, maybe this should work for you:

test=*# select * from test_select;
 id | is_active
----+-----------
  5 | t
  5 | f
  6 | t
  7 | t
  8 | t
  5 | t
  8 | f
  9 | t
 10 | t
  6 | f
 10 | f
(11 rows)

test=*# select id, is_active from (select id, is_active, max(ctid), row_number() over (partition by id) from test_select group by id, is_active, ctid order by id, ctid desc) foo where is_active and row_number = 1;
 id | is_active
----+-----------
  5 | t
  7 | t
  9 | t
(3 rows)


You *should* consider an additional column, timestamp for instance, the
trick with ctid isn't a clean solution.


Andreas
-- 
Andreas Kretschmer
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