MySQL学习笔记之数据定义表约束,分页方法总结

2024-07-24 13:10:35

字体：大中小

来源：转载

供稿：网友

本文实例讲述了MySQL学习笔记之数据定义表约束，分页方法。分享给大家供大家参考，具体如下：

1. primary key 主键

特点：主键是用于唯一标识一条记录的约束，一张表最多只能有一个主键，不能为空也不能重复

create table user1(id int primary key,name varchar(32));mysql> insert into user1 values(1,'hb');Query OK, 1 row affected (0.10 sec)mysql> insert into user1 values(1,'hb');ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'mysql> insert into user1 (name) values('hb');ERROR 1364 (HY000): Field 'id' doesn't have a default value

2. auto_increament 自增长

mysql> create table user2(id int primary key auto_increment,name varchar(34));mysql> insert into user2 (name ) values ("name1");Query OK, 1 row affected (0.09 sec)mysql> insert into user2 (name ) values ("name2");Query OK, 1 row affected (0.05 sec)mysql> insert into user2 (name ) values ("name3");Query OK, 1 row affected (0.13 sec)mysql> select * from user2;+----+-------+| id | name |+----+-------+| 1 | name1 || 2 | name2 || 3 | name3 |+----+-------+

3. unique 唯一约束

特点：表的某列值不能重复，可以添加重复的NULL

create table user3(id int primary key auto_increment,name varchar(34) unique);mysql> create table user3(id int primary key auto_increment,name varchar(34) unique);Query OK, 0 rows affected (0.39 sec)mysql> insert into user3 (name ) values ("name3");Query OK, 1 row affected (0.11 sec)mysql> insert into user3 (name ) values ("name3");ERROR 1062 (23000): Duplicate entry 'name3' for key 'name'

允许插入null，并且可以多个

mysql> insert into user3 (name ) values (null);Query OK, 1 row affected (0.12 sec)mysql> insert into user3 (name ) values (null);Query OK, 1 row affected (0.12 sec)mysql> select * from user3;+----+-------+| id | name |+----+-------+| 3 | NULL || 4 | NULL || 1 | name3 |+----+-------+

4. not null

mysql表的列默认情况下可以为null,如果不允许某列为空则可以使用not null说明

create table user4 (id int primary key auto_increment,name varchar(32) not null);mysql> insert into user4 (name) values(null);ERROR 1048 (23000): Column 'name' cannot be null

5. foreign key 外键

从理论上说先建立主表，再建立从表

雇员表：

create table dept(id int primary key , name varchar(32));

部门表：

create table emp(id int primary key ,name varchar(32),deptid int,constraint myforeignkey foreign key(deptid) references dept(id));mysql> select * from dept;+----+-------+| id | name |+----+-------+| 1 | name1 |+----+-------+1 row in set (0.00 sec)mysql> insert into emp values(1,'aaa',1);Query OK, 1 row affected (0.22 sec)mysql> insert into emp values(1,'aaa',2);ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'mysql> insert into emp values(1,'aaa',null);ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'mysql> insert into emp values(2,'aaa',null);Query OK, 1 row affected (0.13 sec)mysql> select * from emp;+----+------+--------+| id | name | deptid |+----+------+--------+| 1 | aaa |   1 || 2 | aaa |  NULL |+----+------+--------+2 rows in set (0.00 sec)

总结：

① 外键只能指向主表的主见列或者unique
② 外键的数据类型应该与它指向的列类型一致
③ 外键的值：NULL 或者指向列中存在的值
④ 外键可以指向本表的主键列或者unique

mysql 不支持check

create table user99(age int check(age>13));mysql> create table user99(age int check(age>13));Query OK, 0 rows affected (0.19 sec)mysql> insert into user99 values(99);Query OK, 1 row affected (0.04 sec)mysql> select * from user99;+------+| age |+------+|  99 |+------+

mysql 分页

基本语法：

select * from 表明 where 条件 limit 从第几条取,取出几条
mysql 是从第0条开始取数据

mysql> select * from student;+------+--------+---------+---------+------+| id  | name  | chinese | english | math |+------+--------+---------+---------+------+|  1 | 张小明   |   89 |   78 |  90 ||  2 | 李进    |   67 |   98 |  56 ||  3 | 王五    |   87 |   78 |  77 ||  4 | 李一   |   88 |   98 |  90 ||  5 | 李来财    |   82 |   84 |  67 ||  6 | 张进宝   |   55 |   85 |  45 ||  7 | 张小明   |   75 |   65 |  30 |+------+--------+---------+---------+------+7 rows in set (0.05 sec)mysql> select * from student limit 2,2;+------+------+---------+---------+------+| id  | name | chinese | english | math |+------+------+---------+---------+------+|  3 | 王五   |   87 |   78 |  77 ||  4 | 李一  |   88 |   98 |  90 |+------+------+---------+---------+------+2 rows in set (0.00 sec)

按照语文成绩排序，查处第3条到第5条

mysql> select * from student order by chinese desc limit 3,2;+------+--------+---------+---------+------+| id  | name  | chinese | english | math |+------+--------+---------+---------+------+|  5 | 李来财    |   82 |   84 |  67 ||  7 | 张小明   |   75 |   65 |  30 |+------+--------+---------+---------+------+2 rows in set (0.00 sec)

扩展，分页:pageNow , pageSize

select * from 表明 where 条件 [group by … having … order by …]limit 从第几条取,取出几条
select * from 表明 where 条件 [group by … having … order by …]limit (pageNow-1)*pageSize, pageSize

希望本文所述对大家MySQL数据库计有所帮助。

注：相关教程知识阅读请移步到MYSQL教程频道。

上一篇：最新MySQL数据库漏洞情况通报

下一篇：MySQL学习笔记之创建、删除、修改表的方法