How can I select rows corresponding to the unique pair of column values with the highest value of another column in PostgreSQL?

Question

My table looks like this:

A	B	X
1	1	1
1	1	2
1	1	3
1	2	1
1	2	2
2	2	1
2	2	2
2	2	3

I need to select the row with the highest value in X column for each unique A, B pair.

The result would be:

A	B	X
1	1	3
1	2	2
2	2	3

Answer 1

You can use the MAX aggregate function as follows:

select A, B, MAX(X) AS X
  from YOUR_TABLE
group by A, B

Answer 2

I would recommend distinct on :

select distinct on (a, b) t.*
from t
order by a, b, x desc;

This allows you to select other columns from the rows, other than a , b , and x .

With an index on (a, b, x desc) , this would typically be the fastest solution.

Answer 3

That would work like that:

select * from a where x = (select max(x) from a)