Re: count distinct and group by

[Szymon Guz <mabewlun@xxxxxxxxx>]

On 7 May 2015 at 12:39, Magnus Hagander <magnus@xxxxxxxxxxxx> wrote:

On Thu, May 7, 2015 at 12:23 PM, Szymon Guz <mabewlun@xxxxxxxxx> wrote:

Hi,

I'm not sure why there is a reason for such behaviour.

For this table:

create table bg(id serial primary key, t text);

This works:

select count(id) from bg;

This works:

select count(distinct id) from bg;

And this doesn't:

select count(distinct id) from bg order by id;

ERROR:  column "bg.id" must appear in the GROUP BY clause or be used in an aggregate function

LINE 1: select count(distinct id) from bg order by id;

There is no "id" column in the returned dataset to order by. You are just returning one value, how would it be ordered? (and that row has a column named "count" - but you can alias it to SELECT count(distinct id) AS id FROM bg ORDER BY id - it just makes no sense to order a single row.. 

Oh, right. Thanks. I haven't noticed that there is no id column in the dataset.

thanks,

Szymon