分组查询

语法：SELECT 列名 FROM 表名 WHERE 条件 GROUP BY 分组依赖(列);

# 查询各个部门总人数
# 按照部门分组
SELECT department_id, COUNT(department_id)
FROM t_employees
GROUP BY department_id;
# 计算各个部门的薪资平均值
SELECT department_id, AVG(salary)
FROM t_employees
GROUP BY department_id;
# 查询各个部门、各个岗位人数
SELECT department_id, job_id, COUNT(employee_id)
FROM t_employees
GROUP BY department_id, job_id