select后面

仅仅支持标量子查询

-- 案例：查询每个部门的员工个数
SELECT d.*,(
    SELECT COUNT(*)
    FROM employees e
    WHERE e.department_id = d.`department_id`
) 个数
FROM departments d;
-- 案例2：查询员工号=102的部门名
SELECT (
    SELECT department_name
    -- , e.department_id 此处报错，非标量子查询
    FROM departments d
    INNER JOIN employees e
    ON d.department_id=e.department_id
    WHERE e.employee_id=102    
) 部门名;