Re: SELECT syntax question - combining COUNT and DISTINCT - Mailing list pgsql-novice

From paul@entropia.co.uk
Subject Re: SELECT syntax question - combining COUNT and DISTINCT
Date
Msg-id 3F7AA4A6.6216.38E0D25@localhost
Whole thread Raw
In response to SELECT syntax question - combining COUNT and DISTINCT  (Cath Lawrence <Cath.Lawrence@anu.edu.au>)
List pgsql-novice
On 1 Oct 2003 at 17:26, Cath Lawrence wrote:

Does this help?

 select count(distinct pdb_id),chainid from chain group by chain_id;

> I'm having some trouble working with aggregates. I think I'm missing
> the concept of how aggregates are treated. Is there a tutorial page?
>
> Here's my problem, no doubt very simple. It's one table only, nice and
> easy...
> SELECT DISTINCT pdb_id FROM chain WHERE chain_id = 'A';
>
> But now I want to count how many records are returned and I can't work
> it out. I'm sure I'm missing something obvious.
> For instance "SELECT COUNT(pdb_id) FROM chain WHERE chain_id = 'A';"
> works fine but of course is no longer distinct.
>
>
> A secondary question (relating to my quality control problem but not
> the subject heading) is why I can't do this:
> "SELECT DISTINCT chain_id FROM chain WHERE pdb_id IN (SELECT pdb_code
> FROM pdb_entry WHERE chain_count=1);"
> This looks OK but takes apparently forever (or until I lose patience)
> on my Mac G4. There are about 10,000 values in that subquery, so I
> didn't expect it to be lightning fast, but it seems ridiculous.
>
>
> Relevant snippets of table:
> pdb_entry - 19,587 entries
> ---------
> pdb_code is KEY, CHAR(4), unique, indexed
> chain_count is INTEGER
>
> chain - 40,844 entries
> -----
> pdb_id is CHAR(4) Foreign key, indexed
> chain_id is CHAR(1)
>
>
>
> Cath Lawrence,                       Cath.Lawrence@anu.edu.au
> Senior Scientific Programmer,  Centre for Bioinformation Science,
> John Curtin School of Medical Research (room 4088)
> Australian National University,  Canberra ACT 0200
> ph: (02) 61257959   mobile: 0421-902694   fax: (02) 61252595
>
>
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