Consider
create table pairs ( number a, number b )
Where the data is
1,1
1,1
1,1
2,4
2,4
3,2
3,2
5,1
Etc.
What query gives me the distinct values the number column b has So I can see
1,1
5,1
2,4
3,2
only
I've tried
select distinct ( a ) , b from pairs group by b
but gives me "not a group by expression"