Hi,

How to overcome this problem..

1. The best way would be using Foreign key instead of triggers.
Oracle referential integrity provides the same functionality as your trigger.
2. If you insist on using triggers, then remove statement causing error.
If you are trying to delete a row and your trigger is BEFORE delete, you can be sure that in the table REF_COST_CENTRE is at least one row with COST_CENTRE_CODE and COUNTRYCODE you write in WHERE clause - the row you are deleting.
Thus, you don't need to test it.

Regards,
Ales