mysql select后面的子查询

select后面的子查询使用,只能是标量查询

#案例：查询每个部门的员工个数

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

#案例：查询员工号=102的部门名

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

用子查询

SELECT (

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