Back mysql select sub-query

Subqueries use the back of select, only scalar query

Case #: Query the number of employees in each department

SELECT d.*,(
    SELECT COUNT(*)
    FROM employees
    WHERE employees.`department_id`=d.department_id
)
FROM
    departments d;

Case #: Query department employee ID = 102 name

SELECT department_name
FROM
    employees e
INNER JOIN
    departments d
ON
    e.`department_id`=d.`department_id`
WHERE
    e.`employee_id`=102;

Sub-query

SELECT (

SELECT department_name
FROM
    employees e
INNER JOIN
    departments d
ON
    e.`department_id`=d.`department_id`
WHERE
    e.`employee_id`=102
);