本文例項講述了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：用於保證資料的完整性和一致性

主要分為

PRIMARY KEY (PK) #標識該欄位為該表的主鍵，可以唯一的標識記錄
FOREIGN KEY (FK) #標識該欄位為該表的外來鍵
NOT NULL #標識該欄位不能為空
UNIQUE KEY (UK) #標識該欄位的值是唯一的，
AUTO_INCREMENT #標識該欄位的值自動增長（整數型別，而且為主鍵）

DEFAULT #為該欄位設定預設值
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,'sale');
#第二種建立unique的方式
create table department(
  id int,name char(10),unique(id),unique(name)
);
insert into department values(1,'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

# 建立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),-> (2,81),-> (3,25',80);
Query OK,3 rows affected (0.01 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from services;
+------+---------------+------+
| id  | ip      | port |
+------+---------------+------+
|  1 | 192,23 |  80 |
|  2 | 192,23 |  81 |
|  3 | 192,25 |  80 |
+------+---------------+------+
3 rows in set (0.00 sec)
mysql> insert into services values (4,80);
ERROR 1062 (23000): Duplicate entry '192,23-80' for key 'ip'

auto_increment

約束:約束的欄位為自動增長，約束的欄位必須同時被key約束

不指定id，則自動增長

# 建立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',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，會在之前的最後一條記錄繼續增長
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)

對於自增的欄位，在用delete刪除後，再插入值，該欄位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

檢視可用的 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,預設為1
# 起始的偏移量auto_increment_offset,預設是1
# 設定步長 為會話設定，只在本次連線中有效
set session auto_increment_increment=5;
#全域性設定步長 都有效。
set global auto_increment_increment=5;
# 設定起始偏移量
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的值大於auto_increment_increment的值，則auto_increment_offset的值會被忽略

設定完起始偏移量和步長之後，再次執行show variables like'auto_inc%';

發現跟之前一樣，必須先exit,再登入才有效。

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，新增的資料，仍然是以刪除前的最後一樣作為起始。

truncate table t1;資料量大，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複寫這些部門，太冗餘了

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.建立表時先建立被關聯表，再建立關聯表
# 先建立被關聯表（dep表）
create table dep(
  id int primary key,name varchar(20) not null,descripe varchar(20) not null
);
#再建立關聯表（emp表）
create table emp(
  id int primary key,age int not null,dep_id int,constraint fk_dep foreign key(dep_id) references dep(id) //建立約束
);
#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'),'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),'lisi',19,'egon',20,2),(4,'yuanhao',40,3),(5,'alex',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後，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,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
# 趕緊去檢視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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