Using GROUP BY and RAND()

Hi people :)

I'm having a problem with a request using ORDER BY RAND() and GROUP BY
:

SELECT a.id_a, b.id_b
FROM table_A a, table_B b
GROUP BY a.id_a
ORDER BY RAND()

The result I'd like to get would be a table with all my distincts
"id_a" from "table_A", associated, each one, with a random value of
"id_b" contained in my table "Table_B"

I apologize for my english :).

If you have an idea ... thanks :)