本文實例講述了MySQL學習筆記之數據定義表約束，分頁方法。分享給大家供大家參考，具體如下：
1. primary key 主鍵
特點：主鍵是用于唯一標識一條記錄的約束，一張表最多只能有一個主鍵，不能為空也不能重復
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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 自增長
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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 唯一約束
特點：表的某列值不能重復，可以添加重復的NULL
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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' |
允許插入null，并且可以多個
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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表的列默認情況下可以為null,如果不允許某列為空則可以使用not null說明
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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 外鍵
從理論上說先建立主表，再建立從表
雇員表：
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create table dept(id int primary key , name varchar(32)); |
部門表：
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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) |
總結：
① 外鍵只能指向主表的主見列或者unique
② 外鍵的數據類型應該與它指向的列類型一致
③ 外鍵的值：NULL 或者 指向列中存在的值
④ 外鍵可以指向本表的主鍵列或者unique
mysql 不支持check
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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 分頁
基本語法：
select * from 表明 where 條件 limit 從第幾條取,取出幾條
mysql 是從第0條開始取數據
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mysql> select * from student;+------+--------+---------+---------+------+| id | name | chinese | english | math |+------+--------+---------+---------+------+| 1 | 張小明 | 89 | 78 | 90 || 2 | 李進 | 67 | 98 | 56 || 3 | 王五 | 87 | 78 | 77 || 4 | 李一 | 88 | 98 | 90 || 5 | 李來財 | 82 | 84 | 67 || 6 | 張進寶 | 55 | 85 | 45 || 7 | 張小明 | 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) |
按照語文成績排序，查處第3條到第5條
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mysql> select * from student order by chinese desc limit 3,2;+------+--------+---------+---------+------+| id | name | chinese | english | math |+------+--------+---------+---------+------+| 5 | 李來財 | 82 | 84 | 67 || 7 | 張小明 | 75 | 65 | 30 |+------+--------+---------+---------+------+2 rows in set (0.00 sec) |
擴展，分頁:pageNow , pageSize
select * from 表明 where 條件 [group by … having … order by …]limit 從第幾條取,取出幾條
select * from 表明 where 條件 [group by … having … order by …]limit (pageNow-1)*pageSize, pageSize
希望本文所述對大家MySQL數據庫計有所幫助。
