GROUP BY

$PDO = new PDO("mysql:host=mysql;dbname=php-test;charset=utf8","root","root");

$stm = $PDO->query("SELECT department_id, COUNT(*), MAX(salary) FROM employees GROUP BY department_id");
$res = $stm->fetchAll(PDO::FETCH_ASSOC);

echo "<pre>";
print_r($res);
echo "</pre>";

Вернёт количество сотрудников и максимальную зарплату в каждом подразделении

Array
(
    [0] => Array
        (
            [department_id] => 4
            [COUNT(*)] => 3
            [MAX(salary)] => 830
        )

    [1] => Array
        (
            [department_id] => 2
            [COUNT(*)] => 5
            [MAX(salary)] => 1100
        )

    [2] => Array
        (
            [department_id] => 1
            [COUNT(*)] => 11
            [MAX(salary)] => 1000
        )

    [3] => Array
        (
            [department_id] => 3
            [COUNT(*)] => 4
            [MAX(salary)] => 3200
        )

)