Hi...
Use this Query with Decode function
SELECT P.*
FROM PTABLE P,MTABLE M,NTABLE R
WHERE P.MATERIALID = M.MATERIALID
AND P.MATERIALID = N.MATERIALID
AND P.MATERIAL = DEOCDE(P.MATERIAL,NULL,P.MATERIAL,N.MATERIAL)
If you have any clarification,please don't hesitate to contact me. Thank you.