Postgres query to combine multiple rows with same value of a column into a single row

Question

I have this table:

and would like to convert it to the following:

Please help me, been stuck on it for way too long. Doesn't working for me using group by

Answer 1

WITH A as (SELECT id, a FROM XXX WHERE a is not null),
     B as (SELECT id, b FROM XXX WHERE b is not null)
SELECT A.a, B.b, A.id FROM A 
     INNER JOIN B on A.id = B.id;

Answer 2

For this dataset, simple aggregation would do what you want:

select min(a) a, min(b) b, id
from mytable
group by id

This takes advantage of the fact that aggregate functions ignore null values; we could get the very same result with max() as we did with min() .