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     categ1        X2        Y3        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