[打卡]Aloe0424 --聚合函数02

-- GROUP BY
# 查询各个部门的平均工资和最高工资
SELECT department_id,AVG(salary),MAX(salary)
FROM employees
GROUP BY department_id;
# 查询各个工种的平均工资
SELECT job_id,AVG(salary)
FROM employees
GROUP BY job_id;

# GROUP BY 后字段的顺序，代表着分组的顺序，但无论如何分组，结果是一样的
SELECT department_id,job_id,AVG(salary)
FROM employees
GROUP BY department_id,job_id;

SELECT department_id,job_id,AVG(salary)
FROM employees
GROUP BY job_id,department_id;
# 以下写法是错误的，但在Mysql中不会报错
SELECT department_id,job_id,AVG(salary)
FROM employees
GROUP BY department_id;
# 因此,1.SELECT 中出现的非组函数字段必须声明在 GROUP BY 中，反之， GROUP BY 中声明的字段可以不出现在 SELECT中
# 2. GROUP BY 声明在 FROM 后面，也在 WHERE 的后面，在 ORDER BY 的前面，也在 LIMIT 的前面