ABAP query for matching material in another table

Hi Experts,

I have to do auto GR through an interface by calling MB01 for that i have written a BDC and the application is calling that. but i have to put a restriction that it should only do it for few materials that are in ztable "Zbulkmaterial" it has only one field matnr. I need to match materials in this field with the matial in EKPO, but i cant match it directrly, i have to track that in such way :-

slip_no from "zval_wb" (this table dsnt have PO number), then slip_no1(diff than 1st slip no.) from "zallow_sl3"(this table has PO number) by that PO we can get material No from EKPO, and then i have to match it with zbulkmaterial-matnr

As i am new i dnt no what will be the code for this please tel me the query i have to write, is it involves 3 tables i m not able to do that matching of materials.

Regards,

Nikhil