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
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
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希望本文所述对大家MySQL数据库计有所帮助。
