五 .數據庫(表的完整性約束)

時間 2019-11-08

原文原文鏈接

一 .表的完整約束

1.約束簡介

2. NOT NULL(非空)

是否可空，null表示空，非字符串

not null - 不可空

null - 可空

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    |       |
+-------+---------+------+-----+---------+-------+
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)

2 . DEFAULT+NOT NULL（非空+默認值）

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。

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 不生效

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"

3. UNIQUE（惟一約束，指定某列或者幾列組合不能重複）

UNIQUE 約束惟一標識數據庫表中的每條記錄。

UNIQUE 和 PRIMARY KEY 約束均爲列或列集合提供了惟一性的保證。

PRIMARY KEY 擁有自動定義的 UNIQUE 約束。
 請注意，每一個表能夠有多個 UNIQUE 約束，可是每一個表只能有一個 PRIMARY KEY 約束

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 + NOT NULL 結合

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    |       |
+-------+---------+------+-----+---------+-------+
row in set (0.00 sec)

UNIQUE 聯合惟一(表示在這個數據表添加的字段值是惟一 重複添加會報錯)
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'

4. 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和 unique差很少）

============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)
);

mysql> desc department1;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | NO   | PRI | NULL    |       |
| name    | varchar(20)  | NO   | UNI | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
rows in set (0.01 sec)

#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)
);

mysql> desc department2;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | NO   | PRI | NULL    |       |
| name    | varchar(20)  | YES  |     | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
rows in set (0.00 sec)

#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

mysql> desc department3;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | NO   | PRI | NULL    |       |
| name    | varchar(20)  | YES  |     | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
rows in set (0.01 sec)

# 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> desc department4;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | YES  |     | NULL    |       |
| name    | varchar(20)  | YES  |     | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
rows in set (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

mysql> desc department4;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | NO   | PRI | NULL    |       |
| name    | varchar(20)  | YES  |     | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
rows in set (0.01 sec)

==================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> desc service;
+--------------+-------------+------+-----+---------+-------+
| Field        | Type        | Null | Key | Default | Extra |
+--------------+-------------+------+-----+---------+-------+
| ip           | varchar(15) | NO   | PRI | NULL    |       |
| port         | char(5)     | NO   | PRI | NULL    |       |
| service_name | varchar(10) | NO   |     | NULL    |       |
+--------------+-------------+------+-----+---------+-------+
rows in set (0.00 sec)

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'

5. 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> 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    |                |
+-------+-----------------------+------+-----+---------+----------------+
mysql> insert into student(name) values
    -> ('nick'),
    -> ('tank')
    -> ;

mysql> select * from student;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  1 | nick | male |
|  2 | tank | 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 | nick | male   |
|  2 | tank | 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('nick');
Query OK, 1 row affected (0.01 sec)

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

6. FOREING KEY（一個表中的 FOREIGN KEY 指向另外一個表中的 PRIMARY KEY）

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

mysql> desc departments;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| dep_id   | int(4)      | YES  |     | NULL    |       |
| dep_name | varchar(11) | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
rows in set (0.00 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> desc departments;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| dep_id   | int(4)      | NO   |     | NULL    |       |
| dep_name | varchar(11) | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
rows in set (0.00 sec)

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> desc departments;                                                                                                       +----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| dep_id   | int(4)      | YES  | UNI | NULL    |       |
| dep_name | varchar(11) | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
rows in set (0.01 sec)

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)

#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,'jason',1),
(2,'oscar',2),
(3,'nick',2),
(4,'tank',2),
(5,'mac',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 | jason      |      1 |
|  5 | mac     |      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 | jason      |      1 |
|  5 | mac     |      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)