mysql（三）
数据准备
create table student(
number int UNIQUE KEY auto_increment,
name varchar(20) UNIQUE KEY,
klass int not null,
age int not null,
gradName varchar(10)
)auto_increment=201804001;
insert into student(name, klass,age, gradName) value
( '刘一', 19, 16, '二年级'),
( '陈二', 18, 19, '一年级'),
( '张三', 19, 20, '二年级'),
( '李四', 19, 17, '一年级'),
( '王五', 19, 18, '三年级'),
( '赵六', 18, 24, '二年级'),
( '孙七', 19, 22, '三年级'),
( '周八', 19, 21, '二年级'),
( '吴九', 18, 25, '一年级'),
( '郑十', 19, 23, '一年级'),
( '小周周', 18, 20, '二年级'),
( '周周周', 19, 21, '三年级');筛选条件
- 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
~~~mysql
范围查询
select * from student where age between 17 and 20; #找出age在这个范围的字段
select * from student where age in (17, 20); #找出age属于这里面的字段
模糊查询
select * from student where name like ‘周%’; #%匹配任意个字符
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;
select * from student order by age ; #按照age从小到大排序
select * from student order by age desc; #按照age从大到小排序
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 |
~mysql
select count(*) from student; #统计有几条数据
select max(age) from student; #求年龄最大值
select avg(age) from student; #求平均年龄 对于字符字段求值为0
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
select gradName from student group by gradName;
#查看总共有几个年级
select gradName, count(name)as count from student group by gradName;
#查找每个年级有多少人
select gradName, GROUP_CONCAT(name) from student group by gradName;
#查看每个年级有哪些人
select gradName,name from student group by gradName;
#出现其他字段，报错
select age, gradName from student group by age,gradName;
#group by可以分组多个字段，
select gradName, count(number) from student group by gradName with rollup;
#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
#聚合过滤 having
select gradName, count(number) as count
from student
group by gradName
having count(gradName)>3 [order by gradName];
#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
#where不可以使用别名， having可以使用
select age from student where age>20; #查看age>20的字段
select age as new from student where new>20; #错误 不能使用别名，
select age as new from student where age>20; #使用原来的名字可以
select age as new from student having new>20; #having可以使用别名
select age as new from student having age>20; #也可以使用原来的名字
#where不能操作聚合函数， having可以
select gradName, count(age) from student group by gradName; #查看每个年级有多少人
select gradName, count(age)
from student
where count(age)>3
group by gradName;
#报错， where不能操作聚合函数
select gradName, count(age)
from student
group by gradName
having count(age)>3;
#having可以操作聚合函数
#where和having组合使用
select gradName, count(age)
from student
where age>18 group by gradName;
#统计每个年级age>18的有多少人
select gradName, count(age)
from student
where age>18
group by gradName
having gradName='一年级';
#统计一年级age>18的有多少人限制与分页
select * from student limit 5; #从开始查找五条数据;
select * from student limit 0,5; #索引从头开始， 取几个数据
n = 1 # 第几页
m = 5 #一页显示五条
select * from student limit (n-1)*m, m;
作业
统计出每个年级分别有多少人
select gradName as 年级,count(*) as 人数
from student
group by gradName;
统计出每个年级age大于18的人数
select gradName as 年级,count(*) as 人数
from student
where age>18
group by gradName;
统计出一年级的人数
select gradName as 年级,count(*) as 人数
from student
where gradName='一年级';
统计出一年级age大于18的人数
select gradName as 年级,count(*) as 人数
from student
where gradName='一年级' and age>18;