MySQL 小知识点总结
MySQL是最流行的关系型数据库管理系统，在web应用方面，MySQL是最好的RDBMS（关系数据库管理系统）应用软件之一
数据库是按照数据结构来组织，存储和管理数据的仓库
每个数据库都有一个或多个不同的api用于创建，访问，管理，搜索和复制所保存的数据
1.查找mysql里面有多少数据库 show databases;
2.创建自己的数据库 create database student;
3.查看自己的数据库有没有创建出来 show databases;
4.定位到自己创建的数据库 use student;
5.查看当前的数据库里面有没有表格（数据表） show tables;
mysql> use student;//定位到自己创建的数据库
mysql> show tables;
| Tables_in_student |
| user |
1 row in set (0.00 sec)
6.开始创建表create table user(
格式：列 列的类型 是否为空
id int not null,
name varchar(20) not null,
sex int not null,
age int not null,
tel varchar(20) not null,
email varchar(20) not null,
address varchar(20) not null
);
7. 创建完表 开始给表里面添加数据
1.单行插入数据
insert into user (id,name,sex,age,tel,email,address)
values
(1,"张三",0,18,"13767677867","[email protected]","西安市碑林区")；
2.多行插入
insert into user (id,name,sex,age,tel,email,address)
values
(1,"张三",0,18,"13767677867","[email protected]","西安市碑林区")，
(2,"张三",0,18,"13767677867","[email protected]","西安市碑林区")，
(3,"张三",0,18,"13767677867","[email protected]","西安市碑林区");
8.查当前表里面有没有数据
select * from user;
条件查询：
select * from user where id=2;
9.数据表的增删改查
查： select * from user;
删： delete from user; ( 指的是删除表里面的所有数据)
delete from user where id=2 (根据条件删除)
改： update user set name="马六"; 没有条件修改整个数据表里面的整个name列
update user set name="王尼玛" where id=1;根据条件来修改
增： insert into user (id,name,sex,age,tel,email,address)
values
(1,"张三",0,18,"13767677867","[email protected]","西安市碑林区")，
(2,"张三",0,18,"13767677867","[email protected]","西安市碑林区")，
(3,"张三",0,18,"13767677867","[email protected]","西安市碑林区");
还有单行输入。。。。。
10.给表添加主键
一种是在创建表的时候添加 （primary key）
删除表的语法：drop table name；
mysql> create table score(
-> id int primary key,
-> chinese varchar(20) not null,
-> english varchar(20) not null,
-> math varchar(20) not null,
-> history varchar(20) not null);
Query OK, 0 rows affected (0.09 sec)
mysql> drop table score;
Query OK, 0 rows affected (0.02 sec)
一种是在表外添加Alter table score add primary key(id);
删除主键 alter table score drop primary key;
mysql> create table score(
-> id int,
-> chinese varchar(20) not null,
-> english varchar(20) not null,
-> math varchar(20) not null,
-> history varchar(20) not null);
Query OK, 0 rows affected (0.05 sec)
mysql> alter table score add primary key(id);
Query OK, 0 rows affected (0.16 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> alter table score drop primary key;
Query OK, 0 rows affected (0.15 sec)
Records: 0 Duplicates: 0 Warnings: 0
11.添加外键：
foreign key(id) references user(id);
Alter table score add foreign key(id) references user(id);
删除外键： alter table score drop foreign key(id);
mysql> alter table user add primary key(id);//先设置主键
mysql> drop table score;//先删除表
mysql> create table score(
-> id int,
-> foreign key(id) references user(id),
-> chinese varchar(20) not null,
-> english varchar(20) not null,
-> math varchar(20) not null,
-> history varchar(20) not null);
//下面代码测试primary key(id)是否为主键
//主键要求不能重复，不能为空
mysql> insert into user(id,name,sex,age,tel,email,address)
-> values
-> (2,"老李",1,18,"13878738728","[email protected]","咸阳武功");
mysql> insert into user(id,name,sex,age,tel,email,address)
-> values
-> (2,"老李",1,18,"13878738728","[email protected]","咸阳武功");
ERROR 1062 (23000): Duplicate entry '2' for key 'PRIMARY'
mysql> insert into user(id,name,sex,age,tel,email,address)
-> values
-> (,"老李",1,18,"13878738728","[email protected]","咸阳武功");
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that
corresponds to your MySQL server version for the right syntax to use near '"老李
",1,18,"13878738728","[email protected]","咸阳武功")' at line 3
//下面插入表格，foreign key(id) references user(id),因为score的ID是user的id的外键，所以其id只能添加user的id范围内的
mysql> insert into score(id,chinese,english,math,history)
-> values
-> (4,"50","12","147","2");
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint f
ails (`student`.`score`, CONSTRAINT `score_ibfk_1` FOREIGN KEY (`id`) REFERENCES
`user` (`id`))
mysql> insert into score(id,chinese,english,math,history)
-> values
-> (1,"50","12","147","2");
Query OK, 1 row affected (0.01 sec)
mysql> insert into score(id,chinese,english,math,history)
-> values
-> (2,"99","150","147","40");
Query OK, 1 row affected (0.08 sec)
mysql> insert into score(id,chinese,english,math,history)
-> values
-> (3,"10","120","107","140");
Query OK, 1 row affected (0.00 sec)
12.添加自增列
auto_increment必须要求该列是主键(或别的键
mysql> create table teacher(
-> id int primary key auto_increment,//id不仅是主键还是自增列
-> userid int,
-> foreign key(userid) references user(id),//userid是user-id的外键
-> tname varchar(20));
Query OK, 0 rows affected (0.04 sec)
mysql> insert into teacher(id,userid,tname)
-> values
-> (1,1,"毛豆");
Query OK, 1 row affected (0.02 sec)
mysql> insert into teacher(id,userid,tname)
-> values
-> (1,2,"毛豆");//主键不能重复,不能为空
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'
mysql> insert into teacher(userid,tname)//id是自增列
-> values
-> (2,"毛豆");
Query OK, 1 row affected (0.00 sec)
mysql> insert into teacher(userid,tname)
-> values
-> (3,"老吴");
Query OK, 1 row affected (0.01 sec)
//表格显示 user score teacher
mysql> select * from user;
| id | name | sex | age | tel | email | address
| 1 | 王麻子 | 0 | 18 | 13732132131 | [email protected] | 西安市莲湖区 |
| 2 | 老李 | 1 | 18 | 13878738728 | [email protected] | 咸阳武功 |
| 3 | 马六 | 0 | 18 | 13732132131 | [email protected] | 西安市莲湖区 |
3 rows in set (0.00 sec)
mysql> select * from score;
+------+---------+---------+------+---------+
| id | chinese | english | math | history |
+------+---------+---------+------+---------+
| 1 | 50 | 12 | 147 | 2 |
| 2 | 99 | 150 | 147 | 40 |
| 3 | 10 | 120 | 107 | 140 |
+------+---------+---------+------+---------+
3 rows in set (0.01 sec)
mysql> select * from teacher;
+----+--------+--------+
| id | userid | tname |
+----+--------+--------+
| 1 | 1 | 毛豆 |
| 2 | 2 | 毛豆 |
| 3 | 3 | 老吴 |
+----+--------+--------+
3 rows in set (0.00 sec)
mysql> select user.id,user.name,teacher.tname,score.chinese,score.english from user,teacher,score where user.id=score.id and score.id=teacher.userid;
+----+-----------+--------+---------+---------+
| id | name | tname | chinese | english |
+----+-----------+--------+---------+---------+
| 1 | 王麻子 | 毛豆 | 50 | 12 |
| 2 | 老李 | 毛豆 | 99 | 150 |
| 3 | 马六 | 老吴 | 10 | 120 |
+----+-----------+--------+---------+---------+
3 rows in set (0.01 sec)