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_nameFROM employees eINNER JOIN departments dON e.`department_id`=d.`department_id`WHERE e.`employee_id`=102;

用子查询

SELECT (SELECT department_nameFROM employees eINNER JOIN departments dON e.`department_id`=d.`department_id`WHERE e.`employee_id`=102);