Need to create clustered BAPI

hello experts,

I need to create a clustered BAPI. I have two table A and B. i need to get all As as well as Bs on the basis of A. Every B is dependent of A. in other words A has a Collections of Bs. How can i do it ? plz help. If you know the name of any standard BAPI in which above requirement exists then plz provide me name of that BAPI. thks in advance.