二、搭配DISTINCT去重

（以上函数均可）

SELECT SUM(DISTINCT salary) AS 和,AVG(DISTINCT salary) AS 平均,COUNT( DISTINCT salary) AS 去重数量,COUNT(salary) AS 不去重数量
From employees;

三、COUNT()详细介绍

#相当于统计行数方式一
SELECT COUNT(*) 
FROM employees;

#相当于统计行数方式二，其中1可以用其他常量或字段替换
SELECT COUNT(1) 
FROM employees;

效率问题： MYISam存储引擎下，COUNT(*)的效率高 INNODB存储引擎下，COUNT(*)和COUNT(1)的效率差不多，比COUNT(字段)高

因此一般用COUNT(*)统计行数

四、分组查询

#其中[]内为可选
SELECT 分组函数,列表（要求出现在 GROUP BY 的后面）
FROM 表
[WHERE 筛选条件]
GROUP BY 分组列表
[ORDER BY 子句]

示例：

#查询每个工种的最高工资
SELECT MAX(salary) AS 最高工资,job_id
FROM employees
GROUP BY job_id;

#查询每个部门中，邮箱包含a的员工的平均工资（分组前的筛选）
SELECT AVG(salary) AS 平均工资,department_id
FROM employees
WHERE email LIKE '%a%'
GROUP BY department_id;

#查询部门员工数量大于2的部门的员工数量（分组后的筛选）
#使用HAVING
SELECT COUNT(*) AS 员工数量,department_id
FROM employees
GROUP BY department_id
HAVING COUNT(*)>2;

#按照多字段
SELECT COUNT(*) AS 员工数量,job_id,department_id
FROM employees
GROUP BY job_id,department_id;

#完整结构
SELECT AVG(salary) AS 平均工资,department_id
FROM employees
WHERE department_id IS NOT NULL
GROUP BY department_id
HAVING AVG(salary)>9000
ORDER BY AVG(salary) DESC;