MySQL笔记2

显示所有数据库：show databases;
创建数据库：create database 数据库名；
删除数据库：drop database 数据库名；
对数据库进行操作：use 数据库名字；
显示该数据库下的表：show tables;
创建表：create table tablename(id int auto_increment,username varchar(30),primary key(id))；
查看表结构：desc tablename；
删除表中指定id元素： delete from student where id=2;
插入表数据：insert into tablename(username,password,registerdate) value(‘shu’,‘0623’,‘2021-2-3’);
获取该表中的内容（数据）：select * from tablename；
删除表属性：alter table 表名 drop column 属性名字;
在表中添加属性：alter table 表名 add 属性int after password（在password属性后面）;
更改指定表中指定id中某属性的值： update 表名set 属性='266’where id=1;
±—±---------±---------±------±-------------+
| ID | username | password | money | registerdate |
±—±---------±---------±------±-------------+
| 1 | hhj | 456 | 266 | NULL |
| 2 | Wt | 123 | 300 | NULL |
| 3 | 集合 | 0623 | NULL | 2021-06-23 |
±—±---------±---------±------±-------------+
查找大于=255的的数值：select*from users where money>=255;

±—±---------±---------±------±-------------+
| ID | username | password | money | registerdate |
±—±---------±---------±------±-------------+
| 1 | fgd | 456 | 266 | NULL |
| 2 | Wyt | 123 | 300 | NULL |
±—±---------±---------±------±-------------+
2 rows in set (0.01 sec)
限制查找前两行数据：select*from users limit 2;
根据钱 正序排列：select *from mytable order by money;
查找前两行的名字个钱 select username,money from mytable limit 2;
倒序查找排列：select from mytable order by money desc;
前五行正序排列： select from mytable order by money limit 5;
前五行倒序： select *from mytable order by money desc limit 5;
select username,money from mytable order by money desc limit 3;
±---------±------+
| username | money |
±---------±------+
| dhsa | 88 |
| sdh | 66 |
| sish | 56 |
±---------±------+
3 rows in set (0.00 sec)

第二行到第五行排序select *from mytable order by money desc limit 2,5;
显示排序号后的第一行数据： select *from mytable order by money desc limit 1
排序并显示钱数大于三十小于五十：select *from mytable where money >30 and money <80 order by money;
倒序：select from mytable where money >30 and money <80 order by money desc ;
显示不等于sdh的表数据并倒序：select from mytable where username！=‘sdh’ order by money desc;
查找属性中含有‘宋’字的名字（%是多少字符都行）：selec tfrom student where name like ‘宋%’;
查找属性中含有‘宋’字的名字（_是一个_代表一个字符）： selectfrom student where name like ‘宋_’;
distinct去重；

摘选出skill前两个字符：select distinct left(skill,2)from people;
去掉名字相同的：select distinct concat(username)from people;

Id为二的名字有多长：select distinct length(username)from people where id=2;
倒序：select reverse(username)from people where id=2;
更改指定数字：select username,replace(username,‘马’,‘李’)from people where id=16;
只输出表中所有姓名和功能属性：select concat(username,skill)from people;
输出含有某属性的个数：select time ,count(*)from people group by time having time =‘远古时代’;
取余：select mod(7,8);
开根：select sqrt(coin)from people where id=2;
现在日期时间规定好属性名字：select now() as nowtime;
现在日期时间：select now();
现在时间： select curtime();