mysql表的完整性約束

時間 2020-07-26

原文原文鏈接

閱讀目錄

　　概覽
　　not null
　　unique
　　primary key
　　foreign key

概覽

　　爲了防止不符合規範的數據進入數據庫，在用戶對數據進行插入、修改、刪除等操做時，DBMS自動按照必定的約束條件對數據進行監測，使不符合規範的數據不能進入數據庫，以確保數據庫中存儲的數據正確、有效、相容。 html

　　約束條件與數據類型的寬度同樣，都是可選參數，主要分爲如下幾種：mysql

# NOT NULL ：非空約束，指定某列不能爲空； 
# UNIQUE : 惟一約束，指定某列或者幾列組合不能重複
# PRIMARY KEY ：主鍵，指定該列的值能夠惟一地標識該列記錄
# FOREIGN KEY ：外鍵，指定該行記錄從屬於主表中的一條記錄，主要用於參照完整性

NOT NULL

是否可空，null表示空，非字符串
not null - 不可空
null - 可空 nginx

mysql> create table t12 (id int not null);
Query OK, 0 rows affected (0.02 sec)sql

mysql> select * from t12;
Empty set (0.00 sec)數據庫

#不能向id列插入空元素。
mysql> insert into t12 values (null);
ERROR 1048 (23000): Column 'id' cannot be null架構

mysql> insert into t12 values (1);
Query OK, 1 row affected (0.01 sec)ide

not null示例sqlserver

mysql> create table t12 (id int not null);
Query OK, 0 rows affected (0.02 sec)

mysql> select * from t12;
Empty set (0.00 sec)

mysql> desc t12;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)

#不能向id列插入空元素。 
mysql> insert into t12 values (null);
ERROR 1048 (23000): Column 'id' cannot be null

mysql> insert into t12 values (1);
Query OK, 1 row affected (0.01 sec)

DEFAULT

咱們約束某一列不爲空，若是這一列中常常有重複的內容，就須要咱們頻繁的插入，這樣會給咱們的操做帶來新的負擔，因而就出現了默認值的概念。spa

默認值，建立列時能夠指定默認值，當插入數據時若是未主動設置，則自動添加默認值

mysql> create table t13 (id1 int not null,id2 int not null default 222);
Query OK, 0 rows affected (0.01 sec)

mysql> desc t13;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id1 | int(11) | NO | | NULL | |
| id2 | int(11) | NO | | 222 | |
+-------+---------+------+-----+---------+-------+
rows in set (0.01 sec)

# 只向id1字段添加值，會發現id2字段會使用默認值填充
mysql> insert into t13 (id1) values (111);
Query OK, 1 row affected (0.00 sec)

mysql> select * from t13;
+-----+-----+
| id1 | id2 |
+-----+-----+
| 111 | 222 |
+-----+-----+
row in set (0.00 sec)

# id1字段不能爲空，因此不能單獨向id2字段填充值；
mysql> insert into t13 (id2) values (223);
ERROR 1364 (HY000): Field 'id1' doesn't have a default value

# 向id1，id2中分別填充數據，id2的填充數據會覆蓋默認值
mysql> insert into t13 (id1,id2) values (112,223);
Query OK, 1 row affected (0.00 sec)

mysql> select * from t13;
+-----+-----+
| id1 | id2 |
+-----+-----+
| 111 | 222 |
| 112 | 223 |
+-----+-----+
rows in set (0.00 sec)

not null + default 示例

mysql> create table t13 (id1 int not null,id2 int not null default 222);
Query OK, 0 rows affected (0.01 sec)

mysql> desc t13;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id1   | int(11) | NO   |     | NULL    |       |
| id2   | int(11) | NO   |     | 222     |       |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.01 sec)

# 只向id1字段添加值，會發現id2字段會使用默認值填充
mysql> insert into t13 (id1) values (111);
Query OK, 1 row affected (0.00 sec)

mysql> select * from t13;
+-----+-----+
| id1 | id2 |
+-----+-----+
| 111 | 222 |
+-----+-----+
1 row in set (0.00 sec)

# id1字段不能爲空，因此不能單獨向id2字段填充值；
mysql> insert into t13 (id2) values (223);
ERROR 1364 (HY000): Field 'id1' doesn't have a default value

# 向id1，id2中分別填充數據，id2的填充數據會覆蓋默認值
mysql> insert into t13 (id1,id2) values (112,223);
Query OK, 1 row affected (0.00 sec)

mysql> select * from t13;
+-----+-----+
| id1 | id2 |
+-----+-----+
| 111 | 222 |
| 112 | 223 |
+-----+-----+
2 rows in set (0.00 sec)

設置嚴格模式：
不支持對not null字段插入null值
不支持對自增加字段插入」值
不支持text字段有默認值

直接在mysql中生效(重啓失效):
mysql>set sql_mode="STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION";

配置文件添加(永久失效)：
sql-mode="STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION"

not null不生效

UNIQUE

惟一約束，指定某列或者幾列組合不能重複

方法一：
create table department1(
id int,
name varchar(20) unique,
comment varchar(100)
);

方法二：
create table department2(
id int,
name varchar(20),
comment varchar(100),
unique(name)
);

mysql> insert into department1 values(1,'IT','技術');
Query OK, 1 row affected (0.00 sec)
mysql> insert into department1 values(1,'IT','技術');
ERROR 1062 (23000): Duplicate entry 'IT' for key 'name'

unique示例

方法一：
create table department1(
id int,
name varchar(20) unique,
comment varchar(100)
);


方法二：
create table department2(
id int,
name varchar(20),
comment varchar(100),
unique(name)
);


mysql> insert into department1 values(1,'IT','技術');
Query OK, 1 row affected (0.00 sec)
mysql> insert into department1 values(1,'IT','技術');
ERROR 1062 (23000): Duplicate entry 'IT' for key 'name'

方法一：
create table department1(
id int,
name varchar(20) unique,
comment varchar(100)
);

方法二：
create table department2(
id int,
name varchar(20),
comment varchar(100),
unique(name)
);

unique示例

mysql> create table t1(id int not null unique);
Query OK, 0 rows affected (0.02 sec)

mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   | PRI | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)

create table service(
id int primary key auto_increment,
name varchar(20),
host varchar(15) not null,
port int not null,
unique(host,port) #聯合惟一
);

mysql> insert into service values
-> (1,'nginx','192.168.0.10',80),
-> (2,'haproxy','192.168.0.20',80),
-> (3,'mysql','192.168.0.30',3306)
-> ;
Query OK, 3 rows affected (0.01 sec)
Records: 3 Duplicates: 0 Warnings: 0

mysql> insert into service(name,host,port) values('nginx','192.168.0.10',80);
ERROR 1062 (23000): Duplicate entry '192.168.0.10-80' for key 'host'

聯合惟一

PRIMARY KEY

主鍵爲了保證表中的每一條數據的該字段都是表格中的惟一值。換言之，它是用來獨一無二地確認一個表格中的每一行數據。
主鍵能夠包含一個字段或多個字段。當主鍵包含多個欄位時，稱爲組合鍵 (Composite Key),也能夠叫聯合主鍵。
主鍵能夠在建置新表格時設定 (運用 CREATE TABLE 語句)，或是以改變現有的表格架構方式設定 (運用 ALTER TABLE)。
主鍵必須惟一，主鍵值非空；能夠是單一字段，也能夠是多字段組合。

1.單字段主鍵

============單列作主鍵===============
#方法一：not null+unique
create table department1(
id int not null unique, #主鍵
name varchar(20) not null unique,
comment varchar(100)
);

#方法二：在某一個字段後用primary key
create table department2(
id int primary key, #主鍵
name varchar(20),
comment varchar(100)
);

#方法三：在全部字段後單獨定義primary key
create table department3(
id int,
name varchar(20),
comment varchar(100),
primary key(id); #建立主鍵併爲其命名pk_name

# 方法四：給已經建成的表添加主鍵約束
mysql> create table department4(
-> id int,
-> name varchar(20),
-> comment varchar(100));
Query OK, 0 rows affected (0.01 sec)

mysql> alter table department4 modify id int primary key;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0

單字段主鍵

2.多字段主鍵

==================多列作主鍵================
create table service(
ip varchar(15),
port char(5),
service_name varchar(10) not null,
primary key(ip,port)
);

mysql> insert into service values
-> ('172.16.45.10','3306','mysqld'),
-> ('172.16.45.11','3306','mariadb')
-> ;
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0

mysql> insert into service values ('172.16.45.10','3306','nginx');
ERROR 1062 (23000): Duplicate entry '172.16.45.10-3306' for key 'PRIMARY'

多字段主鍵

AUTO_INCREMENT

約束字段爲自動增加，被約束的字段必須同時被key約束

#不指定id，則自動增加
create table student(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') default 'male'
);

mysql> select * from student;
+----+------+------+
| id | name | sex |
+----+------+------+
| 1 | egon | male |
| 2 | alex | male |
+----+------+------+

#也能夠指定id
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 (0.00 sec)

mysql> select * from student;
+----+------+--------+
| id | name | sex |
+----+------+--------+
| 1 | egon | male |
| 2 | alex | male |
| 4 | asb | female |
| 7 | wsb | female |
+----+------+--------+

#對於自增的字段，在用delete刪除後，再插入值，該字段仍按照刪除前的位置繼續增加
mysql> delete from student;
Query OK, 4 rows affected (0.00 sec)

mysql> select * from student;
Empty set (0.00 sec)

mysql> insert into student(name) values('ysb');
mysql> select * from student;
+----+------+------+
| id | name | sex |
+----+------+------+
| 8 | ysb | male |
+----+------+------+

#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 (0.01 sec)

mysql> insert into student(name) values('egon');
Query OK, 1 row affected (0.01 sec)

mysql> select * from student;
+----+------+------+
| id | name | sex |
+----+------+------+
| 1 | egon | male |
+----+------+------+
row in set (0.00 sec)

設置auto_increment

瞭解知識

#在建立完表後，修改自增字段的起始值
mysql> create table student(
-> id int primary key auto_increment,
-> name varchar(20),
-> sex enum('male','female') default 'male'
-> );

mysql> alter table student auto_increment=3;

mysql> show create table student;
.......
ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8

mysql> insert into student(name) values('egon');
Query OK, 1 row affected (0.01 sec)

mysql> select * from student;
+----+------+------+
| id | name | sex |
+----+------+------+
| 3 | egon | male |
+----+------+------+
row in set (0.00 sec)

mysql> show create table student;
.......
ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8

#也能夠建立表時指定auto_increment的初始值，注意初始值的設置爲表選項，應該放到括號外
create table student(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') default 'male'
)auto_increment=3;

#設置步長
sqlserver：自增步長
基於表級別
create table t1（
id int。。。
）engine=innodb,auto_increment=2 步長=2 default charset=utf8

mysql自增的步長：
show session variables like 'auto_inc%';

#基於會話級別
set session auth_increment_increment=2 #修改會話級別的步長

#基於全局級別的
set global auth_increment_increment=2 #修改全局級別的步長（全部會話都生效）

#！！！注意了注意了注意了！！！
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：若是auto_increment_offset的值大於auto_increment_increment的值，則auto_increment_offset的值會被忽略，這至關於第一步步子就邁大了，扯着了蛋
好比：設置auto_increment_offset=3，auto_increment_increment=2

mysql> set global auto_increment_increment=5;
Query OK, 0 rows affected (0.00 sec)

mysql> set global auto_increment_offset=3;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like 'auto_incre%'; #須要退出從新登陸
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 1 |
| auto_increment_offset | 1 |
+--------------------------+-------+

create table student(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') default 'male'
);

mysql> insert into student(name) values('egon1'),('egon2'),('egon3');
mysql> select * from student;
+----+-------+------+
| id | name | sex |
+----+-------+------+
| 3 | egon1 | male |
| 8 | egon2 | male |
| 13 | egon3 | male |
+----+-------+------+

步長:auto_increment_increment,起始偏移量:auto_increment_offset

offset偏移量

FOREIKEY

多表：

假設咱們要描述全部公司的員工，須要描述的屬性有這些：工號姓名部門

公司有3個部門，可是有1個億的員工，那意味着部門這個字段須要重複存儲，部門名字越長，越浪費

解決方法：咱們徹底能夠定義一個部門表而後讓員工信息表關聯該表，如何關聯，即foreign key

mysql> create table departments (dep_id int(4),dep_name varchar(11));
Query OK, 0 rows affected (0.02 sec)

# 建立外鍵不成功
mysql> create table staff_info (s_id int,name varchar(20),dep_id int,foreign key(dep_id) references departments(dep_id));
ERROR 1215 (HY000): Cannot add foreign key

# 設置dep_id非空，仍然不能成功建立外鍵
mysql> alter table departments modify dep_id int(4) not null;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> create table staff_info (s_id int,name varchar(20),dep_id int,foreign key(dep_id) references departments(dep_id));
ERROR 1215 (HY000): Cannot add foreign key constraint

# 當設置字段爲unique惟一字段時，設置該字段爲外鍵成功
mysql> alter table departments modify dep_id int(4) unique;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> create table staff_info (s_id int,name varchar(20),dep_id int,foreign key(dep_id) references departments(dep_id));
Query OK, 0 rows affected (0.02 sec)

創造外鍵的條件

#表類型必須是innodb存儲引擎，且被關聯的字段，即references指定的另一個表的字段，必須保證惟一
create table department(
id int primary key,
name varchar(20) not null
)engine=innodb;

#dpt_id外鍵，關聯父表（department主鍵id），同步更新，同步刪除
create table employee(
id int primary key,
name varchar(20) not null,
dpt_id int,
foreign key(dpt_id)
references department(id)
on delete cascade # 級連刪除
on update cascade # 級連更新
)engine=innodb;

#先往父表department中插入記錄
insert into department values
(1,'教質部'),
(2,'技術部'),
(3,'人力資源部');

#再往子表employee中插入記錄
insert into employee values
(1,'yuan',1),
(2,'nezha',2),
(3,'egon',2),
(4,'alex',2),
(5,'wusir',3),
(6,'李沁洋',3),
(7,'皮卡丘',3),
(8,'程咬金',3),
(9,'程咬銀',3)
;

#刪父表department，子表employee中對應的記錄跟着刪
mysql> delete from department where id=2;
Query OK, 1 row affected (0.00 sec)

mysql> select * from employee;
+----+-----------+--------+
| id | name | dpt_id |
+----+-----------+--------+
| 1 | yuan | 1 |
| 5 | wusir | 3 |
| 6 | 李沁洋 | 3 |
| 7 | 皮卡丘 | 3 |
| 8 | 程咬金 | 3 |
| 9 | 程咬銀 | 3 |
+----+-----------+--------+
rows in set (0.00 sec)

#更新父表department，子表employee中對應的記錄跟着改
mysql> update department set id=2 where id=3;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0

mysql> select * from employee;
+----+-----------+--------+
| id | name | dpt_id |
+----+-----------+--------+
| 1 | yuan | 1 |
| 5 | wusir | 2 |
| 6 | 李沁洋 | 2 |
| 7 | 皮卡丘 | 2 |
| 8 | 程咬金 | 2 |
| 9 | 程咬銀 | 2 |
+----+-----------+--------+
rows in set (0.00 sec)

外鍵操做示例

. cascade方式
在父表上update/delete記錄時，同步update/delete掉子表的匹配記錄

. set null方式
在父表上update/delete記錄時，將子表上匹配記錄的列設爲null
要注意子表的外鍵列不能爲not null

. No action方式
若是子表中有匹配的記錄,則不容許對父表對應候選鍵進行update/delete操做

. Restrict方式
同no action, 都是當即檢查外鍵約束

. Set default方式
父表有變動時,子表將外鍵列設置成一個默認的值但Innodb不能識別

on delete(瞭解)