Re: SELECT DISTINCT ... ORDER BY problem

David Rowley wrote:
>> -----Original Message-----
>> From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-
>> owner@postgresql.org] On Behalf Of Madison Kelly
>> Sent: 08 December 2008 22:19
>> To: pgsql-general@postgresql.org
>> Subject: [GENERAL] SELECT DISTINCT ... ORDER BY problem
>>
>> Hi all,
>>
>>    I've got a table that I am trying to SELECT DISTINCT on one column
>> and ORDER BY on a second column, but am getting the error:
>>
>> SELECT DISTINCT ON expressions must match initial ORDER BY expressions
>>
>>    I can't add the second column to the DISTINCT clause because every
>> row is unique. Likewise, I can't add the first column to my ORDER BY as
>> it'd not sort the way I need it to.
>>
>>    Here is a simplified version of my query:
>>
>> \d table
>>                      Table "table"
>>       Column      |  Type   |                   Modifiers
>>
>> -----------------+---------+----------------------------------------------
>> --
>>   tbl_id          | integer | not null default
>> nextval('tbl_seq'::regclass)
>>   foo             | text    |
>>   bar             | text    |
>>
>> SELECT DISTINCT ON (foo) foo, bar FROM table WHERE bar < '2008-12-07
>> 16:32:46' AND tbl_id=153 ORDER BY bar LIMIT 1;
>>
>
> To make the query valid you would have to ORDER BY foo,bar
> DISTINCT ON in this case is only going to show the first bar value for each
> foo.
>
> Is tbl_id not your PK and only giving 1 row anyway?
>
>>    I understand from:
>>
>> http://archives.postgresql.org/pgsql-sql/2007-02/msg00169.php
>>
>>    That this is not really possible because the any given 'foo' column
>> could match multiple 'bar' columns, so what do you search by? However,
>> it's made some sort of decision as a value is shown in 'bar' for each
>> 'foo'.
>>
>>    So my question is two-fold:
>>
>> 1. Can I not say, somehow, "sort all results by 'bar', and return the
>> first/last 'bar' for each distinct 'foo'?
>>
>> 2. Can I somehow say "Order the results using the value of 'bar' you
>> return, regardless of where it came from"?
>
> You can nest queries:
>
> SELECT foo,bar
> FROM (SELECT DISTINCT ON (foo) foo,
>                                Bar
>       FROM table
>       WHERE bar < '2008-12-07 16:32:46'
>         AND tbl_id=153 ORDER BY foo,bar
> ) AS t ORDER BY bar;
>
> Notice that I'm only applying the final order by in the outer query.
>
> David.

haha, darn...

   I've even done embedded SELECTs before, I should have thought of
that!  Thanks!

Madi