本文實例講述了mysql完整性約束。分享給大家供大家參考，具體如下：
主要內容
- not null 與 default
- unique
- primary
- auto_increment
- foreign key
約束條件作用：用于保證數據的完整性和一致性
主要分為
PRIMARY KEY (PK) #標識該字段為該表的主鍵，可以唯一的標識記錄
FOREIGN KEY (FK) #標識該字段為該表的外鍵
NOT NULL #標識該字段不能為空
UNIQUE KEY (UK) #標識該字段的值是唯一的，
AUTO_INCREMENT #標識該字段的值自動增長（整數類型，而且為主鍵）
DEFAULT #為該字段設置默認值
UNSIGNED #無符號
ZEROFILL #使用0填充
unique
在mysql中稱為單列唯一
#例子1：
create table department(
id int,
name char(10) unique
);
mysql> insert into department values(1,'it'),(2,'it');
ERROR 1062 (23000): Duplicate entry 'it' for key 'name'
#例子2：
create table department(
id int unique,
name char(10) unique
);
insert into department values(1,'it'),(2,'sale');
#第二種創建unique的方式
create table department(
id int,
name char(10) ,
unique(id),
unique(name)
);
insert into department values(1,'it'),(2,'sale');
聯合唯一：只要兩列記錄，有一列不同，既符合聯合唯一的約束
# 創建services表
mysql> create table services(
-> id int,
-> ip char(15),
-> port int,
-> unique(id),
-> unique(ip,port)
-> );
Query OK, 0 rows affected (0.05 sec)
mysql> desc services;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id | int(11) | YES | UNI | NULL | |
| ip | char(15) | YES | MUL | NULL | |
| port | int(11) | YES | | NULL | |
+-------+----------+------+-----+---------+-------+
3 rows in set (0.01 sec)
#聯合唯一，只要兩列記錄，有一列不同，既符合聯合唯一的約束
mysql> insert into services values
-> (1,'192,168,11,23',80),
-> (2,'192,168,11,23',81),
-> (3,'192,168,11,25',80);
Query OK, 3 rows affected (0.01 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from services;
+------+---------------+------+
| id | ip | port |
+------+---------------+------+
| 1 | 192,168,11,23 | 80 |
| 2 | 192,168,11,23 | 81 |
| 3 | 192,168,11,25 | 80 |
+------+---------------+------+
3 rows in set (0.00 sec)
mysql> insert into services values (4,'192,168,11,23',80);
ERROR 1062 (23000): Duplicate entry '192,168,11,23-80' for key 'ip'
auto_increment
約束:約束的字段為自動增長，約束的字段必須同時被key約束
不指定id，則自動增長
# 創建student
create table student(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') default 'male'
);
mysql> desc student;
+-------+-----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-----------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
| sex | enum('male','female') | YES | | male | |
+-------+-----------------------+------+-----+---------+----------------+
rows in set (0.17 sec)
#插入記錄
mysql> insert into student(name) values ('老白'),('小白');
Query OK, 2 rows affected (0.01 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> select * from student;
+----+--------+------+
| id | name | sex |
+----+--------+------+
| 1 | 老白 | male |
| 2 | 小白 | male |
+----+--------+------+
rows in set (0.00 sec)
指定id的情況
mysql> insert into student values(4,'asb','female');
Query OK, 1 row affected (0.00 sec)
mysql> insert into student values(7,'wsb','female');
Query OK, 1 row affected (0.01 sec)
mysql> select * from student;
+----+--------+--------+
| id | name | sex |
+----+--------+--------+
| 1 | 老白 | male |
| 2 | 小白 | male |
| 4 | asb | female |
| 7 | wsb | female |
+----+--------+--------+
rows in set (0.00 sec)
# 再次插入一條不指定id的記錄，會在之前的最后一條記錄繼續增長
mysql> insert into student(name) values ('大白');
Query OK, 1 row affected (0.00 sec)
mysql> select * from student;
+----+--------+--------+
| id | name | sex |
+----+--------+--------+
| 1 | 老白 | male |
| 2 | 小白 | male |
| 4 | asb | female |
| 7 | wsb | female |
| 8 | 大白 | male |
+----+--------+--------+
rows in set (0.00 sec)
對于自增的字段，在用delete刪除后，再插入值，該字段仍按照刪除前的位置繼續增長
mysql> delete from student;
Query OK, 5 rows affected (0.00 sec)
mysql> select * from student;
Empty set (0.00 sec)
mysql> select * from student;
Empty set (0.00 sec)
mysql> insert into student(name) values('ysb');
Query OK, 1 row affected (0.01 sec)
mysql> select * from student;
+----+------+------+
| id | name | sex |
+----+------+------+
| 9 | ysb | male |
+----+------+------+
row in set (0.00 sec)
#應該用truncate清空表，比起delete一條一條地刪除記錄，truncate是直接清空表，在刪除大表時用它
mysql> truncate student;
Query OK, 0 rows affected (0.03 sec)
mysql> insert into student(name) values('xiaobai');
Query OK, 1 row affected (0.00 sec)
mysql> select * from student;
+----+---------+------+
| id | name | sex |
+----+---------+------+
| 1 | xiaobai | male |
+----+---------+------+
row in set (0.00 sec)
mysql>
auto_increment_increment和 auto_increment_offset
查看可用的 開頭auto_inc的詞
mysql> show variables like 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 1 |
| auto_increment_offset | 1 |
+--------------------------+-------+
rows in set (0.02 sec)
# 步長auto_increment_increment,默認為1
# 起始的偏移量auto_increment_offset, 默認是1
# 設置步長 為會話設置，只在本次連接中有效
set session auto_increment_increment=5;
#全局設置步長 都有效。
set global auto_increment_increment=5;
# 設置起始偏移量
set global auto_increment_offset=3;
強調：If the value of auto_increment_offset is greater than that of auto_increment_increment, the value of auto_increment_offset is ignored.
翻譯：如果auto_increment_offset的值大于auto_increment_increment的值，則auto_increment_offset的值會被忽略
設置完起始偏移量和步長之后，再次執行show variables like'auto_inc%';
發現跟之前一樣，必須先exit,再登錄才有效。
mysql> show variables like'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 5 |
| auto_increment_offset | 3 |
+--------------------------+-------+
rows in set (0.00 sec)
#因為之前有一條記錄id=1
mysql> select * from student;
+----+---------+------+
| id | name | sex |
+----+---------+------+
| 1 | xiaobai | male |
+----+---------+------+
row in set (0.00 sec)
# 下次插入的時候，從起始位置3開始，每次插入記錄id+5
mysql> insert into student(name) values('ma1'),('ma2'),('ma3');
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from student;
+----+---------+------+
| id | name | sex |
+----+---------+------+
| 1 | xiaobai | male |
| 3 | ma1 | male |
| 8 | ma2 | male |
| 13 | ma3 | male |
+----+---------+------+
清空表區分delete和truncate的區別：
delete from t1; #如果有自增id，新增的數據，仍然是以刪除前的最后一樣作為起始。
truncate table t1;數據量大，刪除速度比上一條快，且直接從零開始。
foreign key
理解foreign key
如上圖如果一個公司有很多員工，每個員工都對應一個部門，在填表的時候就會重復寫這些部門，太冗余了
我們可以將它們分離
此時有兩張表，一張是employee表,簡稱emp表（關聯表，也就從表）。一張是department表，簡稱dep表（被關聯表，也叫主表）。
#1.創建表時先創建被關聯表，再創建關聯表
# 先創建被關聯表（dep表）
create table dep(
id int primary key,
name varchar(20) not null,
descripe varchar(20) not null
);
#再創建關聯表（emp表）
create table emp(
id int primary key,
name varchar(20) not null,
age int not null,
dep_id int,
constraint fk_dep foreign key(dep_id) references dep(id) //創建約束
);
#2.插入記錄時，先往被關聯表中插入記錄，再往關聯表中插入記錄
insert into dep values
(1,'IT','IT技術有限部門'),
(2,'銷售部','銷售部門'),
(3,'財務部','花錢太多部門');
insert into emp values
(1,'zhangsan',18,1),
(2,'lisi',19,1),
(3,'egon',20,2),
(4,'yuanhao',40,3),
(5,'alex',18,2);
3.刪除表
#按道理來說，刪除了部門表中的某個部門，員工表的有關聯的記錄相繼刪除。
mysql> delete from dep where id=3;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`db5`.`emp`, CONSTRAINT `fk_name` FOREIGN KEY (`dep_id`) REFERENCES `dep` (`id`))
#但是先刪除員工表的記錄之后，再刪除當前部門就沒有任何問題
mysql> delete from emp where dep_id =3;
Query OK, 1 row affected (0.00 sec)
mysql> select * from emp;
+----+----------+-----+--------+
| id | name | age | dep_id |
+----+----------+-----+--------+
| 1 | zhangsan | 18 | 1 |
| 2 | lisi | 18 | 1 |
| 3 | egon | 20 | 2 |
| 5 | alex | 18 | 2 |
+----+----------+-----+--------+
4 rows in set (0.00 sec)
mysql> delete from dep where id=3;
Query OK, 1 row affected (0.00 sec)
mysql> select * from dep;
+----+-----------+----------------------+
| id | name | descripe |
+----+-----------+----------------------+
| 1 | IT | IT技術有限部門 |
| 2 | 銷售部 | 銷售部門 |
+----+-----------+----------------------+
2 rows in set (0.00 sec)
上面的刪除表記錄的操作比較繁瑣，按道理講，裁掉一個部門，該部門的員工也會被裁掉。其實呢，在建表的時候還有個很重要的內容，叫同步刪除，同步更新
on delete cascade #同步刪除
on update cascade #同步更新
create table emp(
id int primary key,
name varchar(20) not null,
age int not null,
dep_id int,
constraint fk_dep foreign key(dep_id) references dep(id)
on delete cascade #同步刪除
on update cascade #同步更新
);
#再去刪被關聯表(dep)的記錄，關聯表(emp)中的記錄也跟著刪除
mysql> delete from dep where id=3;
Query OK, 1 row affected (0.00 sec)
mysql> select * from dep;
+----+-----------+----------------------+
| id | name | descripe |
+----+-----------+----------------------+
| 1 | IT | IT技術有限部門 |
| 2 | 銷售部 | 銷售部門 |
+----+-----------+----------------------+
2 rows in set (0.00 sec)
mysql> select * from emp;
+----+----------+-----+--------+
| id | name | age | dep_id |
+----+----------+-----+--------+
| 1 | zhangsan | 18 | 1 |
| 2 | lisi | 19 | 1 |
| 3 | egon | 20 | 2 |
| 5 | alex | 18 | 2 |
+----+----------+-----+--------+
4 rows in set (0.00 sec)
#再去更改被關聯表(dep)的記錄，關聯表(emp)中的記錄也跟著更改
mysql> update dep set id=222 where id=2;
Query OK, 1 row affected (0.02 sec)
Rows matched: 1 Changed: 1 Warnings: 0
# 趕緊去查看一下兩張表是否都被刪除了，是否都被更改了
mysql> select * from dep;
+-----+-----------+----------------------+
| id | name | descripe |
+-----+-----------+----------------------+
| 1 | IT | IT技術有限部門 |
| 222 | 銷售部 | 銷售部門 |
+-----+-----------+----------------------+
2 rows in set (0.00 sec)
mysql> select * from emp;
+----+----------+-----+--------+
| id | name | age | dep_id |
+----+----------+-----+--------+
| 1 | zhangsan | 18 | 1 |
| 2 | lisi | 19 | 1 |
| 3 | egon | 20 | 222 |
| 5 | alex | 18 | 222 |
+----+----------+-----+--------+
4 rows in set (0.00 sec)
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