MySQL高级查询

Posted 2020-12-03 卖鱼熊

 1 create table student(
 2     number int UNIQUE KEY auto_increment,  
 3     name varchar(20) UNIQUE KEY, 
 4     klass int not null, 
 5     age int not null,
 6     gradName varchar(10)
 7 )auto_increment=201804001;
 8 
 9 insert into student(name, klass,age, gradName) value
10 ( ‘刘一‘, 19, 16, ‘二年级‘),
11 ( ‘陈二‘, 18, 19, ‘一年级‘),
12 ( ‘张三‘, 19, 20, ‘二年级‘),
13 ( ‘李四‘, 19, 17, ‘一年级‘),
14 ( ‘王五‘, 19, 18, ‘三年级‘),
15 ( ‘赵六‘, 18, 24, ‘二年级‘),
16 ( ‘孙七‘, 19, 22, ‘三年级‘),
17 ( ‘周八‘, 19, 21, ‘二年级‘),
18 ( ‘吴九‘, 18, 25, ‘一年级‘),
19 ( ‘郑十‘, 19, 23, ‘一年级‘),
20 ( ‘小周周‘, 18, 20, ‘二年级‘),
21 ( ‘周周周‘, 19, 21, ‘三年级‘);

 

• 范围查询： BETWEEN a AND b

• 间隔返回： in

• 模糊查询： % 匹配任意个任意的字符

• 模糊查询： _ 匹配一个任意字符

1 #范围查询  
2 select * from student where age between 17 and 20;  #找出age在这个范围的字段
3 select * from student where age in (17, 20);    #找出age属于这里面的字段
4 
5 #模糊查询
6 select * from student where name like ‘周%‘;    #%匹配任意个字符
7 select * from student where name like ‘周_‘;    #匹配一个任意字符

排序/去重

• 排序：  SELECT columns FROM tb_name ORDER BY ord_col_1 [asc/desc]; 

• 去重：  SELECT DISTINCT columns FROM tb_name; 

1 select * from student order by age ;    #按照age从小到大排序
2 select * from student order by age ;    #按照age从大到小排序
3 
4 select distinct gradName from student;    #查看有几个年级

聚合/分组

• 聚合

常用聚合函数	描述
COUNT(column)	统计个数
MAX(column)	最大值
MIN(column)	最小值
SUM(column)	求和
AVG(column)	平均值
GROUP_CONCAT(column)	列出字段全部值

1 select count(*) from student;        #统计有几条数据
2 select max(age) from student;        #求年龄最大值
3 select avg(age) from student;        #求平均年龄        对于字符字段求值为0
4 select group_concat(age) from student;        #显示字段所有值

　　分组 group by 在分组的情况下，只能出现聚合列和分组列

 1 select gradName from student group by  gradName;    
 2 #查看总共有几个年级
 3 
 4 select gradName, count(name)as count from student group by  gradName;    
 5 #查找每个年级有多少人
 6 
 7 select gradName, GROUP_CONCAT(name) from student group by gradName;
 8 #查看每个年级有哪些人
 9 
10 select gradName,name from student group by  gradName;    
11 #出现其他字段，报错
12 
13 select age, gradName from student group by age,gradName;
14 #group by可以分组多个字段，
15 
16 select gradName, count(number) from student group by gradName with rollup;
17 #with rollup 在最后加一行统计

　　聚合过滤 having 对聚合出来的数据进行过滤

1 #聚合过滤 having  
2 select  gradName, count(number) as count from student group by  gradName having count(gradName)>3 [order by gradName];    
3 #查看每个年级有多少人，并过滤掉人数小于等于3的， 如果换成where就会报错

where和 having的区别和组合使用

• where 不可以使用别名， having可以

• where不能操作聚合函数

• where和having组合使用是先执行where筛选数据，最后用having筛选数据

• where要写在group by 前面 having要写在group by 后面

 1 #where不可以使用别名， having可以使用
 2 select age from student where age>20;            #查看age>20的字段
 3 select age as new from student where new>20;    #错误 不能使用别名，
 4 select age as new from student where age>20;    #使用原来的名字可以
 5 
 6 select age as new from student having new>20;    #having可以使用别名
 7 select age as new from student having age>20;    #也可以使用原来的名字
 8 
 9 
10 
11 #where不能操作聚合函数， having可以
12 select gradName, count(age) from student group by gradName;    #查看每个年级有多少人
13 select gradName, count(age) from student where count(age)>3 group by gradName;
14 #报错， where不能操作聚合函数
15 
16 select gradName, count(age) from student group by gradName having count(age)>3;
17 #having可以操作聚合函数
18 
19 
20 
21 
22 #where和having组合使用
23 select gradName, count(age) from student where age>18  group by gradName;
24 #统计每个年级age>18的有多少人
25 
26 select gradName, count(age) from student where age>18  group by gradName having gradName=‘一年级‘;    
27 #统计一年级age>18的有多少人

限制与分页

1 select * from student limit 5;        #从开始查找五条数据;
2 
3 select * from student limit 0,5;        #索引从头开始， 取几个数据
4 
5 n = 1        # 第几页
6 m = 5        #一页显示五条
7 select * from student limit (n-1)*m, m;