How to combine many rows into one row (by concatenation?) ?

Hi.

The table is   table foo( id integer, x integer);  and let the data be
id    x
=====
1     10
1     20
2     20
3     30
3     10

Now I would like to get the data in  a format like this

1 , 10 20
2 , 20
3 , 10 30

where id is the first field, and the second field is the x values sorted.

Is there a simple way to do this?