PLSQL_trigger cannot be complied - plsql

Question

I need to create a trigger that when the last employee of the dept is deleted from emp table, the dept is deleted from the dept table.
emp(empno, ename, deptno)
dept(deptno, dname)
First, I created a procedure that deletes the dept from dept table given a deptno.
CREATE OR REPLACE PROCEDURE del_dept
(v_dno in number)
is
begin
delete from DEPT where deptno = v_dno;
end;
Then I created a trigger that deletes the dept when the last emp in that dept is deleted. I tried to test the trigger by deleting one of the three emps in deptno10, but I got error mesg from command that trigger is invalid and failed re-validation.
create or replace trigger del_dept
after delete on EMP
for each row
DECLARE
emp_count Number;
g_dno Number;
begin
SELECT COUNT(:old.ename) INTO emp_count FROM emp group by deptno;
FOR i IN 1.. emp_count LOOP
IF i = emp_count THEN
del_dept(g_dno);
end if;
END LOOP;
End;

Answer 1

Error trigger is invalid and failed re-validation specifies that yor trigger was created with compilation error hence you were not able to test it. This is not a good idea to write a trigger in your case. I can see few issues in you code. So lets first resolve these issues.
When you write the code below, your trigger would be compiled and would be a valid one. So you will not get the error trigger is invalid and failed re-validation
CREATE OR REPLACE TRIGGER del_dept
AFTER DELETE
ON EMP
FOR EACH ROW
DECLARE
EMP_CNT NUMBER;
BEGIN
SELECT COUNT (1)
INTO emp_cnt
FROM emp
WHERE deptno = :old.deptno;
IF emp_cnt = 0
THEN
---del_dept is your procedure to delete records from dept table
del_dept (:old.deptno);
END IF;
END;
But when you would try to execute it you will further get the error:
ORA-04091: table EMP is mutating, trigger/function may not see it
ORA-06512: at "DEL_DEPT", line 4 ORA-04088: error during execution of
trigger 'DEL_DEPT'
The reason for getting this error is you are trying to select from the table from which you are deleting records and thats not allowed in oracle.
The best way for your case is to write a procedure which would delete the records once the last employee data is deleted from your dept table . See below:
CREATE OR REPLACE PROCEDURE del_dept (dept_no NUMBER)
IS
EMP_CNT NUMBER;
BEGIN
SELECT COUNT (1)
INTO emp_cnt
FROM emp
WHERE deptno = dept_no;
IF emp_cnt = 0
THEN
del_dept (dept_no);
END IF;
END;