How to restrict deletion of all rows in a child table(Ex:all emps in a Dept

Question

I have use case where a table which has a child ID, parent ID and some other columns. I want to restrict deletion of all children of a parent.
I tried a 'before delete' trigger as follows and ended up with a mutating table error-ORA-06512 Any suggestions on how this can be done.
Following is the trigger I tried:
create or replace
TRIGGER C_MODULE_EXEC_SELECT
BEFORE DELETE ON C_MODULE_EXEC_SELECT FOR EACH ROW
DECLARE
rowcnt number;
BEGIN
SELECT COUNT(MODULE_CODE) INTO rowcnt FROM C_MODULE_EXEC_SELECT WHERE EXEC_PLAN_ID=:OLD.EXEC_PLAN_ID AND MODULE_CODE != :OLD.MODULE_CODE;
IF rowcnt = 0 THEN
     RAISE_APPLICATION_ERROR(-20000, 'Cannot remove all subject areas from a execution plan');
END IF;
END;
Thanks,
Sireesha

Answer 1

What you have written is clear but it seems to me your rule, as stated, is not valid. At the point where you insert a parent, there will initially be no children. How is that different from any other parent with no children? Is there a guarantee, after inserting a parent, that a child record will be inserted? In one millisecond? One hour? One-hundred years?
And why is the deletion of the last child record any less valid than the deletion of any other child record? Perhaps the rule should be ... when the last child record is deleted the parent should be deleted too. This would be far easier to enforce.
Delete the children and each time check to see if the parent can be deleted. If an exception is raised then it can't be.