append work area into custom table from database table & check the conditn

hi!!

i have to fetch data from three table cobk, coep & repost1.

coep & cobk are database tables

&

repost1 & repost2 are custom tables.

Actually i have append data in a custom table repost2 & for that i have to check that

if belnr(document number) doesnot yet exists in the custom table repost2..

then i have to move the wa_coep into wa_repost2.

here coep is a database table.

please reply soon.

thank you

amit.