MySQL学习笔记之数据定义表约束,分页方法总结

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。

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