Trigger to update a count based on another table

Question

I am working on a php project for managing employees.
i have two tables employee and department with the department number being a relation between the two. department has an attribute that contains a count of employees based on the deparment number.
So where i'm stuck is that i want the employee count to be automatically inserted, so what do you think i should do

Thanks in advance

Answer 1

Instead of storing and updating the value each time something is changed you should just calculate it when needed.
You can use count to do it.

Example

SELECT department_number, count(*) FROM employee GROUP BY department_number

Answer 2

If you don't want to use (or have access to) a trigger, but want to abstract the logic, you could use a view ( docs ).

So assuming the below data, and the idea of having the entire department table and the employee count dynamically calculated, you could have the following query:

CREATE TABLE
  `employees`
(
   `employeeID` INT PRIMARY KEY AUTO_INCREMENT,
   `name` VARCHAR(50),
   `department_number` INT
);

CREATE TABLE
  `departments`
(
  `department_number` INT PRIMARY KEY AUTO_INCREMENT,
  `name` VARCHAR(50)
);

INSERT INTO 
   `departments`
(`department_number`, `name`)
VALUES
(1, 'Tech Department'),
(2, 'Admin Department');

INSERT INTO
  `employees`
(`name`, `department_number`)
VALUES
('John Doe', 1),
('Jane Doe', 1),
('Jimmy Doe', 2);

Then the query:

SELECT
    DepartmentTbl.*,
    DepartmentEmployeeTbl.employee_count
FROM
  `departments` AS DepartmentTbl
LEFT JOIN
  (
   SELECT
      `department_number`,
      COUNT(`employeeID`) AS `employee_count`
    FROM
      `employees`
    GROUP BY
      `department_number`
   ) AS DepartmentEmployeeTbl
   ON DepartmentTbl.department_number = DepartmentEmployeeTbl.department_number 

Gives the result:

department_number | name              | employee_count
-------------------------------------------------------
1                 | Tech Department   | 2
2                 | Admin Department  | 1

SQLFiddle - http://sqlfiddle.com/#!9/3e0b54/1

So to create this view, you could use:

CREATE OR REPLACE VIEW `departments_employee_count` AS
SELECT
        DepartmentTbl.*,
        DepartmentEmployeeTbl.employee_count
    FROM
      `departments` AS DepartmentTbl
    LEFT JOIN
      (
       SELECT
          `department_number`,
          COUNT(`employeeID`) AS `employee_count`
        FROM
          `employees`
        GROUP BY
          `department_number`
       ) AS DepartmentEmployeeTbl
       ON DepartmentTbl.department_number = DepartmentEmployeeTbl.department_number 

Then you could call the view as:

SELECT
    *
FROM
    `departments_employee_count`