[firebird-support] how to combine a field value while their id is same?

[ibmcom2011]

now i have a table such as:

f1       f2
1        aa
2        tf
1        bb
3        cc
2        ht

i want the result seems as:

f1       f2
1        aa, bb
2        tf, ht
3        cc

any help is appreciated,  thanks.