MySql数据库基本操作三（分组查询）

DQL语言学习（Data Query Languge）

表数据链接: https://pan.baidu.com/s/1uDxNjKVKlz9uVicf9IYCkg 提取码: 2y1w

二、分组函数

  功能：用作统计使用，又称为聚合函数或统计函数或组函数
  分类：sum、avg、max、min、count

SELECT SUM(salary)
FROM employees;

SELECT AVG(salary)
FROM employees;

SELECT MAX(salary)
FROM employees;

SELECT MIN(salary)
FROM employees;

SELECT COUNT(salary)
FROM employees;

SELECT SUM(salary) 和,AVG(salary) 平均,MAX(salary) 最高, MIN(salary) 最低,COUNT(salary) 个数
FROM employees;

和distinc搭配

SELECT SUM(DISTINCT salary),SUM(salary) 
FROM employees;
SELECT COUNT(DISTINCT salary),COUNT(salary) 
FROM employees;

COUNT()函数

#统计个数
SELECT COUNT(salary)
FROM employees;
SELECT COUNT(*)
FROM employees;
#等价*
SELECT COUNT(1)
FROM employees;

分组查询

  语法：
    SELECT 分组函数，列（要求出现在group by的后面）
    FROM 表
    WHERE 筛选条件
    GROUP BY 分组的列表
    ORDER BY 子句

  特点：
    分组查询做条件要放在having字句中
    group by子句支持放个字段分组、多个字段分组
    order by放在整个分组查询后面

  案例:：查询每个工种的最高工资

SELECT MAX(salary),job_id
FROM employees
GROUP BY job_id;

  案例：查询每个位置上的部门个数

SELECT COUNT(*),location_id
FROM departments
GROUP BY location_id;

  案例：查询又想中包含a字符的，每个部门的平均工资

SELECT AVG(salary),department_id
FROM employees
WHERE email LIKE '%a%'
GROUP BY department_id;

  案例：有奖金的每个领导手下员工的最高工资

SELECT MAX(salary),manager_id
FROM employees
WHERE commission_pct is NOT NULL
GROUP BY manager_id;

  案例：那个部门的员工个数>2

SELECT COUNT(*),department_id
FROM employees
GROUP BY department_id
HAVING COUNT(*)>2;

  案例6：
    查询每个工种有奖金的员工的最高工资>12000的工种编号和最高工资
    查询每个工种有奖金的员工的最高工资

SELECT MAX(salary),job_id
FROM employees
WHERE commission_pct IS NOT NULL
GROUP BY job_id;

    最高工资大于12000

SELECT MAX(salary),job_id
FROM employees
WHERE commission_pct IS NOT NULL
GROUP BY job_id
HAVING MAX(salary)>12000;

  案例：查询领导编号>102的每个领导手下的最低工资>5000的领导编号是哪个，以及最低工资

SELECT MIN(salary),manager_id
FROM employees
WHERE manager_id>102
GROUP BY manager_id
HAVING MIN(salary)>5000;

按表达式或函数分组

  案例：按员工名的长度分组，查询每一组员工个数，筛选员工个数>5的有哪些

SELECT COUNT(*),LENGTH(last_name) len_name
FROM employees
GROUP BY len_name
HAVING COUNT(*)>5;

按多个字符分组
  案例：查询每个部门每个工种的员工的平均工资

SELECT AVG(salary),department_id,job_id
FROM employees
GROUP BY department_id,job_id;

  添加排序
  查询每个部门每个工种的员工的平均工资,并按照平均工资的高低显示

SELECT AVG(salary),department_id,job_id
FROM employees
GROUP BY department_id,job_id
HAVING AVG(salary)>10000
ORDER BY AVG(salary) DESC;