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mysql完整性約束實(shí)例詳解

本文實(shí)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(nèi)容

not null 與 default
unique
primary
auto_increment
foreign key

約束條件作用：用于保證數(shù)據(jù)的完整性和一致性

主要分為

PRIMARY KEY (PK)    #標(biāo)識(shí)該字段為該表的主鍵，可以唯一的標(biāo)識(shí)記錄
FOREIGN KEY (FK)    #標(biāo)識(shí)該字段為該表的外鍵
NOT NULL    #標(biāo)識(shí)該字段不能為空
UNIQUE KEY (UK)    #標(biāo)識(shí)該字段的值是唯一的，
AUTO_INCREMENT    #標(biāo)識(shí)該字段的值自動(dòng)增長(zhǎng)（整數(shù)類(lèi)型，而且為主鍵）
DEFAULT    #為該字段設(shè)置默認(rèn)值
UNSIGNED #無(wú)符號(hào)
ZEROFILL #使用0填充

unique

在mysql中稱(chēng)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(chuàng)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(lián)合唯一：只要兩列記錄，有一列不同，既符合聯(lián)合唯一的約束

# 創(chuàng)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(lián)合唯一，只要兩列記錄，有一列不同，既符合聯(lián)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(dòng)增長(zhǎng)，約束的字段必須同時(shí)被key約束

不指定id，則自動(dòng)增長(zhǎng)

# 創(chuàng)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(huì)在之前的最后一條記錄繼續(xù)增長(zhǎng)
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(duì)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(xù)增長(zhǎng)

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(yīng)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(shí)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(kāi)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(zhǎng)auto_increment_increment,默認(rèn)為1
# 起始的偏移量auto_increment_offset, 默認(rèn)是1
# 設(shè)置步長(zhǎng) 為會(huì)話設(shè)置，只在本次連接中有效
set session auto_increment_increment=5;
#全局設(shè)置步長(zhǎng) 都有效。
set global auto_increment_increment=5;
# 設(shè)置起始偏移量
set global auto_increment_offset=3;

強(qiáng)調(diào)：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(huì)被忽略

設(shè)置完起始偏移量和步長(zhǎng)之后，再次執(zhí)行show variables like'auto_inc%';

發(fā)現(xiàn)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?yàn)橹坝幸粭l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(shí)候，從起始位置3開(kāi)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(qū)分delete和truncate的區(qū)別：

delete from t1; #如果有自增id，新增的數(shù)據(jù)，仍然是以刪除前的最后一樣作為起始。

truncate table t1;數(shù)據(jù)量大，刪除速度比上一條快，且直接從零開(kāi)始。

foreign key

理解foreign key

如上圖如果一個(gè)公司有很多員工，每個(gè)員工都對(duì)應(yīng)一個(gè)部門(mén)，在填表的時(shí)候就會(huì)重復(fù)寫(xiě)這些部門(mén)，太冗余了

我們可以將它們分離

此時(shí)有兩張表，一張是employee表,簡(jiǎn)稱(chēng)emp表（關(guān)聯(lián)表，也就從表）。一張是department表，簡(jiǎn)稱(chēng)dep表（被關(guān)聯(lián)表，也叫主表）。

#1.創(chuàng)建表時(shí)先創(chuàng)建被關(guān)聯(lián)表，再創(chuàng)建關(guān)聯(lián)表
# 先創(chuàng)建被關(guān)聯(lián)表（dep表）
create table dep(
  id int primary key,
  name varchar(20) not null,
  descripe varchar(20) not null
);
#再創(chuàng)建關(guān)聯(lián)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(chuàng)建約束
);
#2.插入記錄時(shí)，先往被關(guān)聯(lián)表中插入記錄，再往關(guān)聯(lián)表中插入記錄
insert into dep values
(1,'IT','IT技術(shù)有限部門(mén)'),
(2,'銷(xiāo)售部','銷(xiāo)售部門(mén)'),
(3,'財(cái)務(wù)部','花錢(qián)太多部門(mén)');
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(lái)說(shuō)，刪除了部門(mén)表中的某個(gè)部門(mén)，員工表的有關(guān)聯(lián)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(dāng)前部門(mén)就沒(méi)有任何問(wèn)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(shù)有限部門(mén)    |
| 2 | 銷(xiāo)售部  | 銷(xiāo)售部門(mén)       |
+----+-----------+----------------------+
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(gè)部門(mén)，該部門(mén)的員工也會(huì)被裁掉。其實(shí)呢，在建表的時(shí)候還有個(gè)很重要的內(nèi)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(guān)聯(lián)表(dep)的記錄，關(guān)聯(lián)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(shù)有限部門(mén)    |
| 2 | 銷(xiāo)售部  | 銷(xiāo)售部門(mén)       |
+----+-----------+----------------------+
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(guān)聯(lián)表(dep)的記錄，關(guān)聯(lián)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(shù)有限部門(mén)    |
| 222 | 銷(xiāo)售部  | 銷(xiāo)售部門(mén)       |
+-----+-----------+----------------------+
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)

更多關(guān)于MySQL相關(guān)內(nèi)容感興趣的讀者可查看本站專(zhuān)題：《MySQL查詢(xún)技巧大全》、《MySQL常用函數(shù)大匯總》、《MySQL日志操作技巧大全》、《MySQL事務(wù)操作技巧匯總》、《MySQL存儲(chǔ)過(guò)程技巧大全》及《MySQL數(shù)據(jù)庫(kù)鎖相關(guān)技巧匯總》

希望本文所述對(duì)大家MySQL數(shù)據(jù)庫(kù)計(jì)有所幫助。

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