目录
分组查询
连接查询
内连接
外连接
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】
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
特点:
1.分组查询中的筛选条件分为两类
数据源 | 位置 | 关键字 | |
分组前筛选 | 原始表 | group by子句的前面 | where |
分组后筛选 | 分组后的结果集 | group by子句的后面 | having |
①分组函数做条件肯定是放在having子句中
②能用分组前筛选的,就优先考虑使用分组前筛选
2.group by子句支持单个字段分组,多个字段分组(多个字段之间用逗号隔开没有顺序要求),表达式或函数(用得少)
3.也可以添加排序(排序放在整个分组查询的最后)
#引入:查询每个部门的员工个数
SELECT COUNT(*) FROM employees WHERE department_id=90;
#1.简单的分组
#案例1:查询每个工种的员工平均工资
SELECT AVG(salary),job_id
FROM employees
GROUP BY job_id;
#案例2:查询每个位置的部门个数
SELECT COUNT(*),location_id
FROM departments
GROUP BY location_id;
#2、可以实现分组前的筛选
#案例1:查询邮箱中包含a字符的 每个部门的最高工资
SELECT MAX(salary),department_id
FROM employees
WHERE email LIKE '%a%'
GROUP BY department_id;
#案例2:查询有奖金的每个领导手下员工的平均工资
SELECT AVG(salary),manager_id
FROM employees
WHERE commission_pct IS NOT NULL
GROUP BY manager_id;
#3、分组后筛选
#案例:查询哪个部门的员工个数>5
#①查询每个部门的员工个数
SELECT COUNT(*),department_id
FROM employees
GROUP BY department_id;
#② 筛选刚才①结果
SELECT COUNT(*),department_id
FROM employees
GROUP BY department_id
HAVING COUNT(*)>5;
#案例2: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
SELECT job_id,MAX(salary)
FROM employees
WHERE commission_pct IS NOT NULL
GROUP BY job_id
HAVING MAX(salary)>12000;
#案例3:领导编号>102的每个领导手下的最低工资大于5000的领导编号和最低工资
manager_id>102
SELECT manager_id,MIN(salary)
FROM employees
GROUP BY manager_id
HAVING MIN(salary)>5000;
#4.添加排序
#案例:每个工种有奖金的员工的最高工资>6000的工种编号和最高工资,按最高工资升序
SELECT job_id,MAX(salary) m
FROM employees
WHERE commission_pct IS NOT NULL
GROUP BY job_id
HAVING m>6000
ORDER BY m ;
#5.按多个字段分组
#案例:查询每个工种每个部门的最低工资,并按最低工资降序
SELECT MIN(salary),job_id,department_id
FROM employees
GROUP BY department_id,job_id
ORDER BY MIN(salary) DESC;
案例:
#1.查询各job_id的员工工资的最大值,最小值,平均值,总和,并按job_id升序
SELECT MAX(salary),MIN(salary),AVG(salary),SUM(salary),job_id
FROM employees
GROUP BY job_id
ORDER BY job_id;
#2.查询员工最高工资和最低工资的差距(DIFFERENCE)
SELECT MAX(salary)-MIN(salary) DIFFRENCE
FROM employees;
#3.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;
#4.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;
#5.选择具有各个job_id的员工人数
SELECT COUNT(*) 个数,job_id
FROM employees
GROUP BY job_id;
# 6.根据创建时间按照日期进行分组
SELECT create_time,count(*) FROM table WHERE create_time >= '2021-12-08 00:00:00'
AND create_time < '2021-12-20 00:00:00' GROUP BY date_format(create_time, '%Y-%m-%d')
连接查询
含义:又称多表查询,当查询的字段来自于多个表时,就会用到连接查询
笛卡尔乘积现象:表1 有m行,表2有n行,结果=m*n行
发生原因:没有有效的连接条件
如何避免:添加有效的连接条件语法:
select 查询列表
from 表1 别名 【连接类型】
join 表2 别名
on 连接条件
【where 筛选条件】
【group by 分组】
【having 筛选条件】
【order by 排序列表】分类:
内连接(★):inner
外连接
左外(★):left 【outer】
右外(★):right 【outer】
全外:full【outer】
交叉连接:cross
内连接
语法:
select 查询列表
from 表1 别名
inner join 表2 别名
on 连接条件;分类:
等值
非等值
自连接特点:
①添加排序、分组、筛选
②inner可以省略
③ 筛选条件放在where后面,连接条件放在on后面,提高分离性,便于阅读
④inner join连接和sql92语法中的等值连接效果是一样的,都是查询多表的交集
#1、等值连接
#案例1.查询员工名、部门名
SELECT last_name,department_name
FROM departments d
JOIN employees e
ON e.`department_id` = d.`department_id`;
#案例2.查询名字中包含e的员工名和工种名(添加筛选)
SELECT last_name,job_title
FROM employees e
INNER JOIN jobs j
ON e.`job_id`= j.`job_id`
WHERE e.`last_name` LIKE '%e%';
#3. 查询部门个数>3的城市名和部门个数,(添加分组+筛选)
#①查询每个城市的部门个数
#②在①结果上筛选满足条件的
SELECT city,COUNT(*) 部门个数
FROM departments d
INNER JOIN locations l
ON d.`location_id`=l.`location_id`
GROUP BY city
HAVING COUNT(*)>3;
#案例4.查询哪个部门的员工个数>3的部门名和员工个数,并按个数降序(添加排序)
#①查询每个部门的员工个数
SELECT COUNT(*),department_name
FROM employees e
INNER JOIN departments d
ON e.`department_id`=d.`department_id`
GROUP BY department_name
#② 在①结果上筛选员工个数>3的记录,并排序
SELECT COUNT(*) 个数,department_name
FROM employees e
INNER JOIN departments d
ON e.`department_id`=d.`department_id`
GROUP BY department_name
HAVING COUNT(*)>3
ORDER BY COUNT(*) DESC;
#5.查询员工名、部门名、工种名,并按部门名降序(添加三表连接)
SELECT last_name,department_name,job_title
FROM employees e
INNER JOIN departments d ON e.`department_id`=d.`department_id`
INNER JOIN jobs j ON e.`job_id` = j.`job_id`
ORDER BY department_name DESC;
#二)非等值连接
#查询员工的工资级别
SELECT salary,grade_level
FROM employees e
JOIN job_grades g
ON e.`salary` BETWEEN g.`lowest_sal` AND g.`highest_sal`;
#查询工资级别的个数>20的个数,并且按工资级别降序
SELECT COUNT(*),grade_level
FROM employees e
JOIN job_grades g
ON e.`salary` BETWEEN g.`lowest_sal` AND g.`highest_sal`
GROUP BY grade_level
HAVING COUNT(*)>20
ORDER BY grade_level DESC;
#三)自连接
#查询员工的名字、上级的名字
SELECT e.last_name,m.last_name
FROM employees e
JOIN employees m
ON e.`manager_id`= m.`employee_id`;
#查询姓名中包含字符k的员工的名字、上级的名字
SELECT e.last_name,m.last_name
FROM employees e
JOIN employees m
ON e.`manager_id`= m.`employee_id`
WHERE e.`last_name` LIKE '%k%';
外连接
应用场景:用于查询一个表中有,另一个表没有的记录
特点:
1、外连接的查询结果为主表中的所有记录
如果从表中有和它匹配的,则显示匹配的值
如果从表中没有和它匹配的,则显示null
外连接查询结果=内连接结果+主表中有而从表没有的记录
2、左外连接,left join左边的是主表
右外连接,right join右边的是主表
3、左外和右外交换两个表的顺序,可以实现同样的效果
4、全外连接=内连接的结果+表1中有但表2没有的+表2中有但表1没有的
#引入:查询男朋友 不在男神表的的女神名
SELECT * FROM beauty;
SELECT * FROM boys;
#左外连接
SELECT b.*,bo.*
FROM boys bo
LEFT OUTER JOIN beauty b
ON b.`boyfriend_id` = bo.`id`
WHERE b.`id` IS NULL;
#案例1:查询哪个部门没有员工
#左外
SELECT d.*,e.employee_id
FROM departments d
LEFT OUTER JOIN employees e
ON d.`department_id` = e.`department_id`
WHERE e.`employee_id` IS NULL;
#右外
SELECT d.*,e.employee_id
FROM employees e
RIGHT OUTER JOIN departments d
ON d.`department_id` = e.`department_id`
WHERE e.`employee_id` IS NULL;
#全外
USE girls;
SELECT b.*,bo.*
FROM beauty b
FULL OUTER JOIN boys bo
ON b.`boyfriend_id` = bo.id;
#交叉连接
SELECT b.*,bo.*
FROM beauty b
CROSS JOIN boys bo;#引入:查询男朋友 不在男神表的的女神名
SELECT * FROM beauty;
SELECT * FROM boys;
#左外连接
SELECT b.*,bo.*
FROM boys bo
LEFT OUTER JOIN beauty b
ON b.`boyfriend_id` = bo.`id`
WHERE b.`id` IS NULL;
#案例1:查询哪个部门没有员工
#左外
SELECT d.*,e.employee_id
FROM departments d
LEFT OUTER JOIN employees e
ON d.`department_id` = e.`department_id`
WHERE e.`employee_id` IS NULL;
#右外
SELECT d.*,e.employee_id
FROM employees e
RIGHT OUTER JOIN departments d
ON d.`department_id` = e.`department_id`
WHERE e.`employee_id` IS NULL;
#全外
USE girls;
SELECT b.*,bo.*
FROM beauty b
FULL OUTER JOIN boys bo
ON b.`boyfriend_id` = bo.id;
#交叉连接
SELECT b.*,bo.*
FROM beauty b
CROSS JOIN boys bo;
案例
#一、查询编号>3的女神的男朋友信息,如果有则列出详细,如果没有,用null填充
SELECT b.id,b.name,bo.*
FROM beauty b
LEFT OUTER JOIN boys bo
ON b.`boyfriend_id` = bo.`id`
WHERE b.`id`>3;
#二、查询哪个城市没有部门
SELECT city
FROM departments d
RIGHT OUTER JOIN locations l
ON d.`location_id`=l.`location_id`
WHERE d.`department_id` IS NULL;
#三、查询部门名为SAL或IT的员工信息
SELECT e.*,d.department_name,d.`department_id`
FROM departments d
LEFT JOIN employees e
ON d.`department_id` = e.`department_id`
WHERE d.`department_name` IN('SAL','IT');
SELECT * FROM departments
WHERE `department_name` IN('SAL','IT');#一、查询编号>3的女神的男朋友信息,如果有则列出详细,如果没有,用null填充
SELECT b.id,b.name,bo.*
FROM beauty b
LEFT OUTER JOIN boys bo
ON b.`boyfriend_id` = bo.`id`
WHERE b.`id`>3;
#二、查询哪个城市没有部门
SELECT city
FROM departments d
RIGHT OUTER JOIN locations l
ON d.`location_id`=l.`location_id`
WHERE d.`department_id` IS NULL;
#三、查询部门名为SAL或IT的员工信息
SELECT e.*,d.department_name,d.`department_id`
FROM departments d
LEFT JOIN employees e
ON d.`department_id` = e.`department_id`
WHERE d.`department_name` IN('SAL','IT');
SELECT * FROM departments
WHERE `department_name` IN('SAL','IT');