1,创建数据库,create datebase 数据库名,在这里kenan建立一个数据库名为myfirstdb
- create database myfirstdb;
2,查看所有的数据库
- show databases;
3,选择数据库，只有先选择数据库，才能在选择的数据库中进行数据库操作
- use myfirstdb;
4,删除数据库
- drop database myfirstdb;
数据库的增删改查
这里要先使用 use来选择要操作的数据库
1，建立数据库表
- use myfirstdb;
- create table user(
- id int auto_increment primary key,
- username varchar(50) not null,
- password varchar(50) not null,
- createtime datetime,
- )
这里创建了一个名为user 的表，id为int型的，auto_increatement表明这个是自增字段,primary key表明这个是主键,username varchar(50)表明userame是可变的字符长，not null表明该字段不能够为空，createtime datetime 字段cratetime是日期时间类型的。
2， 查看表结构
在这里展示了两种查看表结构的方式
- mysql> desc user;
- +------------+-------------+------+-----+---------+----------------+
- | Field | Type | Null | Key | Default | Extra |
- +------------+-------------+------+-----+---------+----------------+
- | id | int(11) | NO | PRI | NULL | auto_increment |
- | username | varchar(50) | NO | | NULL | |
- | password | varchar(50) | NO | | NULL | |
- | createtime | datetime | YES | | NULL | |
- +------------+-------------+------+-----+---------+----------------+
- 4 rows in set (0.01 sec)
- mysql> show columns from user;
- +------------+-------------+------+-----+---------+----------------+
- | Field | Type | Null | Key | Default | Extra |
- +------------+-------------+------+-----+---------+----------------+
- | id | int(11) | NO | PRI | NULL | auto_increment |
- | username | varchar(50) | NO | | NULL | |
- | password | varchar(50) | NO | | NULL | |
- | createtime | datetime | YES | | NULL | |
- +------------+-------------+------+-----+---------+----------------+
- 4 rows in set (0.01 sec)
3,修改表结构
添加列
- mysql> alter table user add age int;
- Query OK, 0 rows affected (0.25 sec)
- Records: 0 Duplicates: 0 Warnings: 0
修改列
注意关键词 modify
- mysql> alter table user modify username varchar(30);
- Query OK, 0 rows affected (0.19 sec)
- Records: 0 Duplicates: 0 Warnings: 0
修改后的表结构
- mysql> desc user;
- +------------+-------------+------+-----+---------+----------------+
- | Field | Type | Null | Key | Default | Extra |
- +------------+-------------+------+-----+---------+----------------+
- | id | int(11) | NO | PRI | NULL | auto_increment |
- | username | varchar(30) | YES | | NULL | |
- | password | varchar(50) | NO | | NULL | |
- | createtime | datetime | YES | | NULL | |
- | age | int(11) | YES | | NULL | |
- +------------+-------------+------+-----+---------+----------------+
- 5 rows in set (0.01 sec)
4,删除表
- mysql> drop table user;
- Query OK, 0 rows affected (0.03 sec)
5,插入数据
这里使用了日期函数，表示当前时间和日期，字符串要用单引号引起来
- mysql> insert into user values(null,'kenan','kenan',now());
- Query OK, 1 row affected (0.13 sec)
6,查询数据
第一句表示查询所有的字段从user表里边，在这里*表示所有，第二句表示单独查询username和password字段
- mysql> select * from user;
- +----+----------+----------+---------------------+
- | id | username | password | createtime |
- +----+----------+----------+---------------------+
- | 1 | kenan | kenan | 2012-10-30 21:46:31 |
- +----+----------+----------+---------------------+
- 1 row in set (0.00 sec)
- mysql> select username,password from user;
- +----------+----------+
- | username | password |
- +----------+----------+
- | kenan | kenan |
- +----------+----------+
- 1 row in set (0.00 sec)
6,带有where字句的条件查询
这里分别进行了三个带有where字句的子查询，分别对id，和username进行限制，用and 连接两个限制条件
- mysql> select * from user where id > 1;
- mysql> select * from user where username = 'kenan';
- mysql> select * from user where username = 'kenan' and id = 1;
7，group by 对查询结果进行分组
这里展示的group by的用法，首先第一句是按照username分组，所以 username相同的数据会认为是一组 只显示第一条数据，第二个sql语句和第三个sql语句用了两个分组函数avg（），sum（）函数，这两个函数 呢，是跟group by 配合使用的，avg（）是用来求这一组数据的平均值，而sum（）是用来求这一组数据的 和。
- mysql> select * from user;
- +----+----------+----------+---------------------+
- | id | username | password | createtime |
- +----+----------+----------+---------------------+
- | 1 | kenan | kenan | 2012-10-30 21:46:31 |
- | 2 | lele | lele | 2012-10-31 15:01:20 |
- | 3 | kenan | lele | 2012-10-31 15:06:46 |
- +----+----------+----------+---------------------+
- 3 rows in set (0.00 sec)
- mysql> select * from user group by username;
- +----+----------+----------+---------------------+
- | id | username | password | createtime |
- +----+----------+----------+---------------------+
- | 1 | kenan | kenan | 2012-10-30 21:46:31 |
- | 2 | lele | lele | 2012-10-31 15:01:20 |
- +----+----------+----------+---------------------+
- 2 rows in set (0.04 sec)
- mysql> select username,avg(id) from user group by username;
- +----------+---------+
- | username | avg(id) |
- +----------+---------+
- | kenan | 2.0000 |
- | lele | 2.0000 |
- +----------+---------+
- 2 rows in set (0.04 sec)
- mysql> select username,sum(id) from user group by username;
- +----------+---------+
- | username | sum(id) |
- +----------+---------+
- | kenan | 4 |
- | lele | 2 |
- +----------+---------+
- 2 rows in set (0.03 sec)
8，order by 对结果进行排序
在这里第一个sql语句，查出所有的user表中的数据，默认是按照你插入时的数据来排序，就是id递增 第二句是 按照username排序，这个会比较username的首字母 order by id 是按照id递增排序 order by id desc 是按照id递减排序，这里大家明白 desc的作用了吧 order by username，id是添加了两个排序条件，首先按照username排序，username相同的话，会按照id排序 在排序中如果遇到NULL值，把NULL值按照最小的值来处理
- mysql> select * from user;
- +----+----------+----------+---------------------+
- | id | username | password | createtime |
- +----+----------+----------+---------------------+
- | 1 | kenan | kenan | 2012-10-30 21:46:31 |
- | 2 | lele | lele | 2012-10-31 15:01:20 |
- | 3 | kenan | lele | 2012-10-31 15:06:46 |
- +----+----------+----------+---------------------+
- 3 rows in set (0.00 sec)
- mysql> select * from user order by username;
- +----+----------+----------+---------------------+
- | id | username | password | createtime |
- +----+----------+----------+---------------------+
- | 1 | kenan | kenan | 2012-10-30 21:46:31 |
- | 3 | kenan | lele | 2012-10-31 15:06:46 |
- | 2 | lele | lele | 2012-10-31 15:01:20 |
- +----+----------+----------+---------------------+
- 3 rows in set (0.00 sec)
- mysql> select * from user order by id;
- +----+----------+----------+---------------------+
- | id | username | password | createtime |
- +----+----------+----------+---------------------+
- | 1 | kenan | kenan | 2012-10-30 21:46:31 |
- | 2 | lele | lele | 2012-10-31 15:01:20 |
- | 3 | kenan | lele | 2012-10-31 15:06:46 |
- +----+----------+----------+---------------------+
- 3 rows in set (0.00 sec)
- mysql> select * from user order by id desc
- -> ;
- +----+----------+----------+---------------------+
- | id | username | password | createtime |
- +----+----------+----------+---------------------+
- | 3 | kenan | lele | 2012-10-31 15:06:46 |
- | 2 | lele | lele | 2012-10-31 15:01:20 |
- | 1 | kenan | kenan | 2012-10-30 21:46:31 |
- +----+----------+----------+---------------------+
- 3 rows in set (0.00 sec)
- mysql> select * from user order by username,id;
- +----+----------+----------+---------------------+
- | id | username | password | createtime |
- +----+----------+----------+---------------------+
- | 1 | kenan | kenan | 2012-10-30 21:46:31 |
- | 3 | kenan | lele | 2012-10-31 15:06:46 |
- | 2 | lele | lele | 2012-10-31 15:01:20 |
- +----+----------+----------+---------------------+
- 3 rows in set (0.00 sec)
9，distinct的用法
distinct的是去重，然后把不重复的查询出来
- mysql> select distinct username from user;
- +----------+
- | username |
- +----------+
- | kenan |
- | lele |
- +----------+
- 2 rows in set (0.00 sec)
10,like模糊查询
在这里用了like模糊查询 第一个sql语句，是查询出来 uername 以 ke 开头的 行 第二个sql语句，是查询出来 username 的第二个字母是e的行 在这里%表示0或多个字符，_代表一个字符
- mysql> select * from user where username like 'ke%';
- +----+----------+----------+---------------------+
- | id | username | password | createtime |
- +----+----------+----------+---------------------+
- | 1 | kenan | kenan | 2012-10-30 21:46:31 |
- | 3 | kenan | lele | 2012-10-31 15:06:46 |
- +----+----------+----------+---------------------+
- 2 rows in set (0.00 sec)
- mysql> select * from user where username like '_e%';
- +----+----------+----------+---------------------+
- | id | username | password | createtime |
- +----+----------+----------+---------------------+
- | 1 | kenan | kenan | 2012-10-30 21:46:31 |
- | 2 | lele | lele | 2012-10-31 15:01:20 |
- | 3 | kenan | lele | 2012-10-31 15:06:46 |
- +----+----------+----------+---------------------+
- 3 rows in set (0.01 sec)
11,LIMIT 限定结果行数，用来进行分页查询
limit的两种用法 第一种limit 1这个是显示查询出来的数据的第一条 当让1换成x，就是x条 第二种limit 1,2这里limit带有的两个参数 首先我们把查询出来的数据进行编号，从0号开始(当然这个编号是不存在的,就是说表内不存在这个字段)，它不是id 这个编号是便于我们理解人为的编号 然后第一个参数代表的含义呢，就是要显示的数据的编号 第二个参数表示的就是从刚刚编号开始（包含选中的编号）显示多少条数据 分页用法，要显示第x页的数据，每页显示y条 首先计算第x也的数据的开始编号是（x-1）*y 所以这个sql语句应该这样写 select * from user limit (x-1)*y,y;
- mysql> select * from user limit 1;
- +----+----------+----------+---------------------+
- | id | username | password | createtime |
- +----+----------+----------+---------------------+
- | 1 | kenan | kenan | 2012-10-30 21:46:31 |
- +----+----------+----------+---------------------+
- 1 row in set (0.00 sec)
- mysql> select * from user limit 1,2;
- +----+----------+----------+---------------------+
- | id | username | password | createtime |
- +----+----------+----------+---------------------+
- | 2 | lele | lele | 2012-10-31 15:01:20 |
- | 3 | kenan | lele | 2012-10-31 15:06:46 |
- +----+----------+----------+---------------------+
- 2 rows in set (0.00 sec)
12,修改记录
这里把user表中的所有username叫做kenan的记录更新createtime字段，更新为当前时间
- mysql> update user set createtime = now() where username = 'kenan';
- Query OK, 2 rows affected (0.05 sec)
- Rows matched: 2 Changed: 2 Warnings: 0
13，删除记录
这里删除user表中所有username叫做lele的记录
- mysql> delete from user where username = 'lele';
- Query OK, 1 row affected (0.06 sec)
今天就写到这里吧，这个是最长的一片博客了，大家有问题可以回复我哦！