Categories and subcategories : more details

> I have two tables : the first for categories and the second for
> subcategories.I want to get a result like Yahoo! For each category ,
> the sql
> query must return maximum 3 subcategories. I can't use "limit" in a
> subquery
> because I use Postgresql 7.0.

Examples :

table CATEG :
ida categ
1 X
2 Y
3 Z

table SUBCATEG :
idb subcateg ida
1 x1 1
2 x2 1
3 x3 1
4 x4 1
5 y1 2
6 y2 2
7 z1 3

I 'd like to obtain a table with maximum 2 subcateg per categ :

table result :

categ subcateg
X x1
X x2
Y y1
Y y2
Z z1

any help?

Regards,

Rachel