Обсуждение: Problems with group by ... order by

I can't figure out why the following doesn't work:

   select
    (case
        when count1 < 300 then 'Other'
        else country1
        end) as country2,
    sum(count1) as count2
    from (select coalesce(country, 'None') as country1, count(*) as count1
            from userProfiles group by country1) as counts1
    group by country2
    order by (country2 = 'Other'), count2 desc

Basically, I want to get an aggregate count of users across countries
(including the "None" country), then I want to aggregate those
countries with counts less than 300 into an Other pseudo-country.  I
want it sorted by this final count, except I want the Other entry to be
last.  This works fine if I leave out the first order-by condition -
the result is sorted by the final count.  But with the query as written
above, I get:

   ERROR:  column "country2" does not exist

It also works fine if I just order by country2, count2 - it seems to be
the comparison that's the problem.  And ordering by a boolean after
aggregating works fine in this simpler case:

   select country, count(*) as cnt
    from userProfiles
    group by country
    order by country is null, cnt;

This puts the NULL count at the end.

This is with PG 7.4.7.  Any advice appreciated, including how to do
this in a simpler fashion.

- John Burger
   MITRE

John D. Burger wrote:
> I can't figure out why the following doesn't work:
>
>   select
>     (case
>         when count1 < 300 then 'Other'
>         else country1
>         end) as country2,
>     sum(count1) as count2
>     from (select coalesce(country, 'None') as country1, count(*) as count1
>             from userProfiles group by country1) as counts1
>     group by country2
>     order by (country2 = 'Other'), count2 desc
>

Do either of these work for you? Note, completely untested, and just off the top
of my head.


select
    (case
        when count1 < 300 then 'Other'
        else country1
        end) as country2,
    sum(count1) as count2
    from (select coalesce(country, 'None') as country1, count(*) as count1
            from userProfiles group by country1) as counts1
    group by country2
    order by (1 = 'Other'), count2 desc


select
    (case
        when count1 < 300 then null
        else country1
        end) as country2,
    sum(count1) as count2
    from (select coalesce(country, 'None') as country1, count(*) as count1
            from userProfiles group by country1) as counts1
    group by country2
    order by (1 is null), count2 desc


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"John D. Burger" <john@mitre.org> writes:
> I can't figure out why the following doesn't work:
>    select
>     (case
>         when count1 < 300 then 'Other'
>         else country1
>         end) as country2,
>     sum(count1) as count2
>     from (select coalesce(country, 'None') as country1, count(*) as count1
>             from userProfiles group by country1) as counts1
>     group by country2
>     order by (country2 = 'Other'), count2 desc

>    ERROR:  column "country2" does not exist

ORDER BY (and also GROUP BY) permit references to output column names
only when they are *unadorned*.  You cannot use them in expressions.

This is a compromise between SQL92 and SQL99 rules ... it's a bit ugly.

            regards, tom lane