[sql] Distinct pair of values SQL

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"