Question

This is a query which selects a set of desired rows:

select max(a), b, c, d, e
from T
group by b, c, d, e;

The table has a primary key, in column id.

I would like to identify these rows in a further query, by getting the primary key from each of those rows. How would I do that? This does not work:

select id, max(a), b, c, d, e
from T 
group by b, c, d, e;

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

I have tried this from poking around in some other postgresql questions, but no luck:

select distinct on (id) id, max(a), b, c, d, e
from T 
group by b, c, d, e;

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

What do I do? I know there can only be one id for each result, cause it’s a primary key… I literally want the primary key along with the rest of the data, for each row that the initial (working) query returns.

Answer 1

If you don’t care which id you get then you just need to wrap your id in some aggregate function that is guaranteed to give you a valid id. The max and min aggregates come to mind:

-- Or min(id) if you want better spiritual balance.
select max(id), max(a), b, c, d, e
from T 
group by b, c, d, e;

---

Depending on your data I think using a window function would be a better plan (thanks to evil otto for the boot to the head):

select id, a, b, c, d, e
from (
    select id, a, b, c, d, e, rank() over (partition by b,c,d,e order by a desc) as r
    from T
) as dt
where r = 1