Mysql增删改查、Mysql查询属性、Mysql使用案例

create database

创建db1数据库

create database db1 charset utf8;

show databases

查看数据库

show databases;

alter database

修改库的编码

alter database db1 charset gdk;

drop database

删除db1数据库

drop database db1;

use databases

使用数据库

use database;

show tabels

查看数据表

show tabels;

select ... where

在stu表中查询sid为1的所有数据

select * from stu where sid =1;

create table

创建一个数据表stu 字段有id(唯一key自增）name字符16个，age不能为空

create table stu(id int primary key auto_increment,name char(16),age int no null);

inset into

在表格插入名字123，年龄10

inset into stu(name,age) value("123",10);

alter table stu rename

修改表格名

alter table stu rename stu_new;

alter table stu modify

修改stu的age字段类型

alter table stu modify age char(4) not null;  

alter table stu add

新增一个字段名

alter table stu add sex enum("男","女");

alter table stu drop

删除一个字段名

alter table stu drop sex;

alter table stu change

修改原有字段名

alter table stu change name  name_new char(16);

desc tabelname

查看数据表的字段属性

desc tabelname;

updata stu set

更新stu表格的age当id为2的数据

updata stu set age=xx  where id=2;

delete from ... where ..

清除表格数据当id为2

delete from stu where id=2;

查询属性

select distinct 字段1，字段2 from 表名
        where 条件
        group by field
        having 筛选条件
        order by filed
        limit 条数
        distinct去重

注：

group by field 根据什么进行分组，一般是某个字段或多个字段

order by field 根据什么进行排序，一般是某个字段或多个字段

having主要配合group by 使用，对分组后的数据进行过滤，里面可以使用 聚合函数

where是针对select查询的过滤，各有区别和用处

优先级：

from

where

group by

select

distinct

having

order by

limit

 

案例：

1、查询姓名末尾的字符

select emp_name,substring(reverse(emp_name),1,1) as new_name from emp;
select emp_name,substring(emp_name,-1) as new_name from emp;

2、查询以“张”开头的三个字员工信息

select * from emp where emp_name like "张__";

3、查询以"z"开头的员工信息

select * from emp where emp_name like "z%";

4、计算每个部门有多少人？大于6个人的部门有哪些？

select post,count(emp_name) 人数 from emp group by post;
select post,count(emp_name) 人数 from emp group by post having count(emp_name)>6;

5、计算每个部门的平均工资从高到低排序(order by desc or asc)

select post,AVG(salary) as avg_salay from emp group by post;
select post,AVG(salary) as avg_salay from emp group by post order by desc;

6、查询成绩第一的

select max(num) from stu;
select * from stu where num = (select max(num) from stu);

7、排除成绩第一的取第二成绩的

select max(num) from stu where num<(select max(num) from stu);
select * from stu where num = (select max(num) from stu where num<(select max(num) from stu));

8、合并第一和第二的数据

select * from stu where num = (select max(num) from stu)
unsion all
select * from stu where num = (select max(num) from stu where num<(select max(num) from stu));