SQL select the max from each group and given them different lables - postgresql

Question

For the following tables:
-- People
id | category | count
----+----------+-------
1 | a | 2
1 | a | 3
1 | b | 2
2 | a | 2
2 | b | 3
3 | a | 1
3 | a | 2
I know that I can find the max count for each id in each category by doing:
SELECT id, category, max(count) from People group by category, id;
With result:
id | category | max
----+----------+-------
1 | a | 3
1 | b | 2
2 | a | 2
2 | b | 3
3 | a | 2
But what if now I want to label the max values differently, like:
id | max_b_count | max_a_count
----+-------------+------------
1 | 2 | 3
2 | 3 | 2
3 | Null | 2
Should I do something like the following?
WITH t AS (SELECT id, category, max(count) from People group by category, id)
SELECT t.id, t.count as max_a_count from t where t.category = 'a'
FULL OUTER JOIN t.id, t.count as max_b_count from t where t.category = 'b'
on t.id;
It looks weird to me.

Answer 1

This is the exact use case why the filter_clause was added to the Aggregate Expressions
With filter_clause you may limit which row you aggregate
aggregate_name ( * ) [ FILTER ( WHERE filter_clause ) ]
Your example
SELECT id,
max(count) filter (where category = 'a') as max_a_count,
max(count) filter (where category = 'b') as max_b_count
from People
group by id
order by 1;
id|max_a_count|max_b_count|
--+-----------+-----------+
1| 3| 2|
2| 2| 3|
3| 2| |

Answer 2

This is one way you can do it:
with T as (select id, category, max(count_ab) maks
from people
group by id, category
order by id)
select t3.id
, (select t1.maks from T t1 where category = 'b' and t1.id = t3.id) max_b_count
, (select t2.maks from T t2 where category = 'a' and t2.id = t3.id) max_a_count
from T t3
group by t3.id
order by t3.id
Here is a demo
Also, as you can see, I have changed the name of the column count to count_ab because it is not a good practice to use keywords as columns names.