Re: Picking the first of an order in an aggregate query

DISTINCT is a very simple solution!
But I have one problem: In addition to the FIRST fields, I also do
want some aggregate functions. More accurately, it would be:

SELECT grouping_field, FIRST(field_a), FIRST(field_b), SUM(field_x),
MAX(field_y)
...

How should I do that? Should I do two queries with a join on the
grouping field? Or is there a more direct way?

On 12/31/12, Jack Christensen <jack(at)jackchristensen(dot)com> wrote:
> On 12/31/2012 8:33 AM, Robert James wrote:
>> I have a query
>>
>> SELECT grouping_field, MIN(field_a), MIN(field_b)
>> FROM ...
>> GROUP BY grouping_field
>>
>> But, instead of picking the MIN field_a and MIN field_b, I'd like to
>> pick field_a and field_b from the first record, according to an order
>> I'll specify. In pseudo-SQL, it would be something like this:
>>
>> SELECT grouping_field, FIRST(field_a), FIRST(field_b)
>> FROM ...
>> ORDER BY field_c DESC, field_d ASC, myfunc(field_e) ASC
>> GROUP BY grouping_field
>>
>> How can I do that with Postgres?
>>
>>
> select distinct on (grouping_field), field_a, field_b
> from ...
> order by grouping_field, field_a asc, field_b asc
>
>
> http://www.postgresql.org/docs/9.2/static/sql-select.html#SQL-DISTINCT
>
> --
> Jack Christensen
> http://jackchristensen.com/
>
>
>
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