Mysql---完整性约束

Posted 2021-01-03 猿自大山深处

完整性约束---用于保证数据的完整性和一致性

KEY POINT:

• not null 与 default
• unique
• primary key
• auto_increment
• foreign key

一 not null 与 default

　　1.是否可空 

　　　　null空 

　　　　not null不可空

　　2.默认值default,当不设置时，默认值为null,当设置默认值时，默认值为设置的默认值

　　　　a.默认值可以为空:

　　　　b.设置not null，插入值时不能为空

　　　　c.设置id字段有默认值后，则无论id字段是null还是not null，都可以插入空，插入空默认填入default指定的默认值

二 unique

　　1.单列唯一

　　2.联合唯一

　　3.组合唯一

三 primary key

　　1.主键  相当于 not null +unique  字段的值唯一且不为空　

mysql> create table t10(
    -> id int primary key,
    -> name char(16)
    -> );
Query OK, 0 rows affected (0.04 sec)

mysql> insert into t10 values
    -> (1,"xiaoma"),
    -> (2,"alex"),
    -> (3,"wusir");
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from t10;
+----+--------+
| id | name   |
+----+--------+
|  1 | xiaoma |
|  2 | alex   |
|  3 | wusir  |
+----+--------+
3 rows in set (0.00 sec)

mysql> insert into t10 values
    -> (1,"tailiang");
ERROR 1062 (23000): Duplicate entry ‘1‘ for key ‘PRIMARY‘
mysql> desc t10;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id    | int(11)  | NO   | PRI | NULL    |       |
| name  | char(16) | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.00 sec)

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　　2.一个表中可以有单列做主键，也可以有多列做主键

mysql> create table t12(
    -> ip char(15),
    -> port int,
    -> primary key(ip,port)
    -> );
Query OK, 0 rows affected (0.05 sec)

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　　3.存储引擎默认是（innodb）:对于innodb存储引擎来说，一张表必须有一个主键。

四 auto_increment  约束的字段为自动增长，约束的字段必须同时被key约束

　　1.不指定id,则自动增长　

mysql> create table t14
    -> (
    -> id int primary key auto_increment,
    -> name varchar(15) not null);
Query OK, 0 rows affected (0.05 sec)

mysql> insert into t14 values ("太白"),("太黑");
ERROR 1136 (21S01): Column count doesn‘t match value count at row 1
mysql> insert into t14(name) values ("太白"),("太黑");
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from t14;
+----+--------+
| id | name   |
+----+--------+
|  1 | 太白   |
|  2 | 太黑   |
+----+--------+
2 rows in set (0.00 sec)

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　　2.也可指定id

　　3.对于自增的字段，在用delete删除后，再插入值，该字段仍按照删除前的位置继续增长

　　4.注意：

　　　　a.步长auto_increment_increment,默认为1

      　　 b.起始的偏移量auto_increment_offset, 默认是1

　　　　可通过设置以上两个值改变默认的步长和起始偏移量

mysql> use db1
Database changed

#创建表t16
mysql> create table t16(
    -> id int primary key auto_increment,
    -> name varchar(10) not null
    -> );
Query OK, 0 rows affected (0.04 sec)

#插入数据
mysql> insert into t16 values
    -> (1,"alex"),
    -> (2,"wusir"),
    -> (3,"egon"),
    -> (4,"xiaomage");
Query OK, 4 rows affected (0.01 sec)
Records: 4  Duplicates: 0  Warnings: 0

#显示t16详细数据
mysql> select * from t16;
+----+----------+
| id | name     |
+----+----------+
|  1 | alex     |
|  2 | wusir    |
|  3 | egon     |
|  4 | xiaomage |
+----+----------+
4 rows in set (0.00 sec)

#删除 id=2的字段记录
mysql> delete from t16 where id=2;
Query OK, 1 row affected (0.00 sec)

mysql> select * from t16;
+----+----------+
| id | name     |
+----+----------+
|  1 | alex     |
|  3 | egon     |
|  4 | xiaomage |
+----+----------+
3 rows in set (0.00 sec)

#删除 id=4的字段记录
mysql> delete from t16 where id=4;
Query OK, 1 row affected (0.01 sec)

mysql> select * from t16;
+----+------+
| id | name |
+----+------+
|  1 | alex |
|  3 | egon |
+----+------+
2 rows in set (0.00 sec)

#插入新的纪录
mysql> insert into t16 values
    -> ("haha");
ERROR 1136 (21S01): Column count doesn‘t match value count at row 1
mysql> insert into t16(name) values
    -> ("haha");
Query OK, 1 row affected (0.01 sec)

mysql> select * from t16;
+----+------+
| id | name |
+----+------+
|  1 | alex |
|  3 | egon |
|  5 | haha |#新插入的字段依然按照之前顺序排列
+----+------+
3 rows in set (0.00 sec)

#查看可用的 开头auto_inc的词
mysql> show variables like ‘auto_inc%‘;
+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| auto_increment_increment | 1     | #步长默认为1
| auto_increment_offset    | 1     |  #偏移量默认为1
+--------------------------+-------+
2 rows in set, 1 warning (0.02 sec)

mysql> set session auto_increment_increment=5; #设置步长为5
Query OK, 0 rows affected (0.01 sec)

mysql> set session auto_increment_offset=2; #设置偏移量为2
Query OK, 0 rows affected (0.00 sec)

#确认设置成功
mysql> show variables like ‘auto_inc%‘;
+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| auto_increment_increment | 5     |
| auto_increment_offset    | 2     |
+--------------------------+-------+
2 rows in set, 1 warning (0.00 sec)

#重新插入新纪录
mysql> insert into t16(name) values
    -> ("灌灌灌灌");
Query OK, 1 row affected (0.00 sec)
#查看新纪录id
mysql> select * from t16;
+----+--------------+
| id | name         |
+----+--------------+
|  1 | alex         |
|  3 | egon         |
|  5 | haha         |
|  7 | 灌灌灌灌     |
+----+--------------+
4 rows in set (0.00 sec)

mysql> insert into t16(name) values
    -> ("丰富");
Query OK, 1 row affected (0.01 sec)

mysql> select * from t16;
+----+--------------+
| id | name         |
+----+--------------+
|  1 | alex         |
|  3 | egon         |
|  5 | haha         |
|  7 | 灌灌灌灌     |
| 12 | 丰富         |
+----+--------------+
5 rows in set (0.00 sec)

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五 foreign key

六 delete 和 truncate的区别