oracle - Trigger Cannot query on same table? -

i want create trigger before insert or update trigger on table customer:

create table customer (custnum number, debitnum number ); 

the validation need in trigger customer not assigned same debitnum twice.

so trigger code is:

create or replace trigger xx_mytrig before insert or update on customer declare ln_count number; begin     select count(custnum) ln_count     customer     custnum:=:new.custnum      , debitnum:=new.debitnum;     if ln_count>0         raise fnd_api.g_exc_error;     end if; end; 

when insert or update noticed select statement not executed. how can query on same table...?

i think can achieve adding unique constraint on columns custnum , debitnum. approach more effective implemented trigger.

alter table customer add constraint constraint_name unique (custnum , debitnum);