group by - Distinct pair of values SQL

Question

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"

Answer 1

What you mean is either

SELECT DISTINCT a, b FROM pairs;

or

SELECT a, b FROM pairs GROUP BY a, b;