Re: 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(at)mitre(dot)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