mysql---基础操作

Posted 2020-08-01

mysql补充

• 创建指定字符编码的数据库

   create database sysdb
   default character set utf8;
  

• 删除数据库

  drop database sysdb;
  

• 查看数据库的默认字符集

  show create database sysdb;
  

• 修改数据库

  alter database sysdb default character set gbk;
  

表管理

    use sysdb;

    show tables; //查看所有表
    //建立数据表
    create table student (
    sid int,
    sname varchar(20),
    sage int
    );

    desc student; //查看表的结构

    drop table student; //删除数据表

• 添加字段

  alter table student add column sgender varchar(2);
  

• 删除字段

   alter table student drop column sgender;
  

• 修改表的字段类型

   alter table student modify column sname varchar(100);
  

• 修改数据表的名字

   alter table student change sname(原名) userName(新名) varchar(20);
  

• 修改表的名称

  alter table student rename to teacher;
  

增加数据

• 插入数据

  insert into student values(1 , ‘cx‘, ‘man‘ , 20);
  

• 插入部分字段

  insert into student(id,name) values(2,‘eric‘);
  

修改数据

• 1.修改单个字段

  update student set gender ="女"  //修改所有
  
  update student set gender ="女" where id = 1;  //修改指定
  

• 2.修改多个字段

  update student set gender = ‘男‘ , age = 30 where id = 1;//逗分隔
  

• 3.删除表中所有数据

  delete from student;
  

• 4.带条件的删除

  delete from student where id = 2;
  

#delete from 和 truncate table 区别

• delete from 可以待条件删除，truncate table 不能带条件删除

• delete from 只能删除表的数据，不能删除表的约束，truncate既可以删除表的数据，也可以删除表的约束

查询数据

• 查询所有列

   select * from student;
  

• 查询指定列

  select id,name from student;
  

• 查询时指定别名（多表查询，别名非常有用）

   select id as ‘编号‘ , name as ‘姓名‘ from student s;//s为表的别名
  

• 查询时添加常量列

  select id , name , gender , age , ‘年级一‘ as  ‘年级‘  from student;
  
  查询时加的列为常量列
  

• 查询时合并列

  select id , name , (成绩一+成绩二) as ‘总成绩’ from student
   合并列只能合并数值类型的列
  

• 查询时去除重复记录

  select distinct sex from stuent;
  //拿到男，女两个结果
  

• 条件查询（where）

  • 逻辑查询 and , or

        //查询id为2，姓名为李四的学生
         select * from  student   where id = 2  and name = ‘李四‘
       //查询id为2， 或者姓名为张三的学生 
         select * from  student   where id = 2  and name = ‘张三‘
    

  • 比较条件 > < >= <= <>(不等于) between and

        select * from student where jsp between 75 and 90
    
        select * from student where sex <> ‘女‘
    

  • 判空 is null / is not null / = ‘ ‘ / <> ‘ ‘

       //查询地址为空的学生
    
       select * from student where address is null; //判断null
       select * from student where address = ‘‘; //判断空
       select * from student where address is null or address = ‘ ‘;           
    
      //查询有地址的学生
       select * from student where address is not null and address <> ‘ ‘;
    

重点

• 模糊条件查询

       %表示匹配n个任意字符
       //查询姓张的学生
       select * from name where name like ‘张%‘
       //查询姓‘李‘,且姓名只有两个字的学生
       ­­_匹配一个任意字符
        select * from name where name like ‘李_‘