分组查询

分组查询

把一张表拆分成多个小组，进行查询得到我们想要的结果

GROUP BY子语句法

可以使用GROUP BY子局将表中的数据分成若干组

SELECT 分组函数，列  （还要求出现在group by的后面）

FROM 表

 【where 条件筛选】

group by 分组的列表

   【order by 子句】

注意：

 查询列表必须特殊，要求是分组函数和group by后出现的字段

简单分组查询练习

1.查询每个部门的平均工资

SELECT AVG(salary) FROM employees；

2.查询每个工种的最高工资

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

3.查询每个位置上的部门个数

SELECT CONT(*),localtion_id
FROM departments
GROUP BY location_id;

添加筛选条件[这种是原始表里就能找到筛选的字段]

1.查询邮箱中包含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

2.查询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;

添加复杂的筛选条件,添加分组后的筛选

1.查询哪个部门的员工个数>2

• 查询每个部门的员工个数

• SELECT CONT(*),department_id 
  FROM employees 
  GROUP BY department_id;

• 根据上一步结果进行筛选，查询哪个部门的员工个数大于2

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

2.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

• 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;

• 根据上一步结果筛选最高工资大于12000

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

3.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

• 查询领导编号>102的每个领导手下的最低工资

SELECT MIN(salary),manager_id
FROM employees
WHERE manager_id >120
GROUP BY manager_id;

• 最低工资>5000的领导编号是哪个

SELECT MIN(salary),manager_id 
FROM employees 
WHERE manager_id >120 
GROUP BY manager_id
HAVING MIN(salary)>5000 ;

按表达式或函数分组

1.按员工姓名的长度分组，查询每一组的员工，筛选员工个数>5的有哪些

• 查询每个长度的员工个数

SELECT COUNT(*),LENGTH(last_name) len_name
FROM employees
GROUP BY LENGTTH(last_name);

• 筛选员工个数>5的有哪些

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

按多个字段分组

1.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 deparment_id,job_id;

添加排序

SELECT AVG(salary),department_id,job_id
FROM employees
WHERE deparment_id IS NOT_NULL
GROUP BY job_id,department_id
ORDER BY AVG(salary) DESC;

练习

1.查询各个管理者手下员工的最低工资，其中最低工资不能低于6000，没有管理者的员工不计算在内

SELECT MIN(salary),manager_id
FROM employees
WHERE manager_id IS NOT NULL
GROUP BY manager_id
HAVING MIN(salary)>=6000;

2.查询所有部门的编号，员工数量和工资平均值，并按平均工资降序

SELECT department_id,COUNT(*),AVG(salary) a
FROM employees
GROUP BY department_id
ORDER BY a DESC;