本文例項講述了MySQL學習筆記之資料定義表約束，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：主鍵是用於唯一標識一條記錄的約束，一張表最多隻能有一個主鍵，不能為空也不能重複

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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：表的某列值不能重複，可以新增重複的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，並且可以多個

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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表的列預設情況下可以為null,如果不允許某列為空則可以使用not null說明

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123	`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 外來鍵

從理論上說先建立主表，再建立從表

僱員表：

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1	`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 外來鍵只能指向主表的主見列或者unique
② 外來鍵的資料型別應該與它指向的列型別一致
③ 外來鍵的值：NULL 或者指向列中存在的值
④ 外來鍵可以指向本表的主鍵列或者unique

mysql 不支援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條開始取資料

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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)

擴充套件，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

更多關於MySQL相關內容感興趣的讀者可檢視本站專題：《MySQL索引操作技巧彙總》、《MySQL日誌操作技巧大全》、《MySQL事務操作技巧彙總》、《MySQL儲存過程技巧大全》、《MySQL資料庫鎖相關技巧彙總》及《MySQL常用函式大彙總》

希望本文所述對大家MySQL資料庫計有所幫助。

MySQL學習筆記之資料定義表約束,分頁方法總結

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