SQL statement question

Hi ALL

I have two tables, table a and table b.

table a
po_no buyer color size
12345 abc blue XXL

table b
po_no buyer color size style
.......... abc blue XXL AAA
.......... abc blue XXL BBB
.......... abc blue XXL CCC

How could I write a SQL statement to lookup table b po_no from table a po_no

Requirement:
Buyer, Color and Size in table a = Buyer, Color and Size in table b

Hugo