Table not updating when trigger is fired - sql

Question

I have 2 tables:
Employees(Employee_ID, First_name, Last_name, Email, Hire_date, Department_ID)
Departments(Department_ID, Department_name, Total_employees).
I have created trigger that fires after inserting an employee to employees table. It increases the total_employees column by 1 for :new inserted Department_ID.
Trigger:
create or replace TRIGGER emp_count_department
AFTER INSERT OR DELETE ON EMPLOYEES
for each row
DECLARE
counter NUMBER;
BEGIN
if inserting then
select total_employees into counter
from departments
where department_id = :new.department_id;
update departments
set total_employees = counter+1
where department_id = :new.department_id;
elsif deleting then
select total_employees into counter
from departments
where department_id = :old.department_id;
update departments
set total_employees = counter-1
where department_id = :old.department_id;
end if;
END;
When I try to insert an employee, it is inserted but it doesn't increase total_employees by 1 in departments table. When I try to insert an employee with a department_ID that doesn't exist in departments table, it gives me error. So the trigger is working but not increasing the total numbers by 1.

Answer 1

Try this:
create or replace TRIGGER emp_count_department
AFTER INSERT OR DELETE ON EMPLOYEES
for each row
BEGIN
if inserting then
update departments
set total_employees = total_employees+1
where department_id = :new.department_id;
elsif deleting then
update departments
set total_employees = total_employees-1
where department_id = :old.department_id;
end if;
END;

Answer 2

I did simple testing of your code in Oracle 11g and it seems to work.
Please check if you missed in adding information.
create table emp1(emp_id number ,dept_id number);
create table dept1 (dept_id number ,total_emp number);
CREATE OR replace TRIGGER emp_count_dept
AFTER INSERT OR DELETE ON emp1
FOR EACH ROW
DECLARE
counter NUMBER;
BEGIN
IF inserting THEN
UPDATE dept1
SET total_emp = total_emp + 1
WHERE dept_id = :new.dept_id;
ELSIF deleting THEN
UPDATE dept1
SET total_emp = total_emp - 1
WHERE dept_id = :old.dept_id;
END IF;
END;
insert into dept1 values(10,10);
insert into emp1 values(10,10);
select * from dept1;
DEPT_ID TOTAL_EMP
10 11

Answer 3

I think you are missing an important use case, what if the employees table is updated and the dept_id changes. To expand upon the answer #pmdba gave, I think the following code would work best.
CREATE OR REPLACE TRIGGER emp_count_dept
AFTER INSERT OR UPDATE OF DEPT_ID OR DELETE ON employees
FOR EACH ROW
BEGIN
IF :new.dept_id IS NOT NULL THEN
UPDATE departments d
SET d.total_employees = NVL(d.total_employees, 0)+1
WHERE d.department_id = :new.dept_id;
END IF;
IF :old.dept_id IS NOT NULL THEN
UPDATE departments d
SET d.total_employees = d.total_employees - 1
WHERE d.department_id = :old.dept_id;
END IF;
END;
/
This takes care of UPDATE statements now. And also takes care of the first INSERT for a given department (If you didn't initialize all departments.total_employee to 0 when you created them).
I also created this DBFiddle demonstrating the functionality (And why you need the NVL function or to default your TOTAL_EMPLOYEES column to 0) (Link)