sql- underlying operating constraints

Constraints include:

• 1. The primary key constraint primary key;
• 2. Non-null constraints not null;
• 3. The only constraint unique;
• 4. foreign key constraints foreign key.

Wherein the primary key constraint comprises a non-empty and the only constraint constraint.

I use the table:

mysql> create table stu(
    ->          id int,
    ->          name varchar(20),
    ->          age int,
    ->          score double(4,2),
    ->          address_id int
    -> );
Query OK, 0 rows affected (0.02 sec)

mysql> create table address(
    ->          id int primary key auto_increment, # id自增
    ->          description varchar(20)
    -> );
Query OK, 0 rows affected (0.02 sec)

insert into address(description)values('France'),('England'),('China');

Non-empty constraint

create:

• Adding NOT NULL constraint when creating tables name varchar(20) not null.
• Created later added alter table stu modify name varchar(20) not null.

delete:

• alter table stu modify name varchar(20)。

example:

创建表以后：
mysql> alter table stu modify name varchar(20) not null;
Query OK, 0 rows affected (0.06 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> desc stu;
+------------+-------------+------+-----+---------+-------+
| Field      | Type        | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| id         | int(11)     | YES  |     | NULL    |       |
| name       | varchar(20) | NO   |     | NULL    |       |
| age        | int(11)     | YES  |     | NULL    |       |
| score      | double(4,2) | YES  |     | NULL    |       |
| address_id | int(11)     | YES  |     | NULL    |       |
+------------+-------------+------+-----+---------+-------+
5 rows in set (0.01 sec)

创建表时：
mysql> create table stu(
    ->          id int,
    ->          name varchar(20) not null,
    ->          age int,
    ->          score double(4,2),
    ->          address_id int)
    -> ;
Query OK, 0 rows affected (0.02 sec)
mysql> desc stu;
+------------+-------------+------+-----+---------+-------+
| Field      | Type        | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| id         | int(11)     | YES  |     | NULL    |       |
| name       | varchar(20) | NO   |     | NULL    |       |
| age        | int(11)     | YES  |     | NULL    |       |
| score      | double(4,2) | YES  |     | NULL    |       |
| address_id | int(11)     | YES  |     | NULL    |       |
+------------+-------------+------+-----+---------+-------+
5 rows in set (0.01 sec)

删除非空约束：
mysql> alter table stu modify name varchar(20);
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> desc stu;
+------------+-------------+------+-----+---------+-------+
| Field      | Type        | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| id         | int(11)     | YES  |     | NULL    |       |
| name       | varchar(20) | YES  |     | NULL    |       |
| age        | int(11)     | YES  |     | NULL    |       |
| score      | double(4,2) | YES  |     | NULL    |       |
| address_id | int(11)     | YES  |     | NULL    |       |
+------------+-------------+------+-----+---------+-------+
5 rows in set (0.01 sec

The primary key constraint

create:

• When you create a table, write directly on the primary key constraint: id int primary keyor re-write the final primary key (id).
• Then add a primary key constraint in the future to create the table: alter table stu modify id int primary keyand alter table stu add primary key (id).
  Note: Use primary key(id)the time, will set the default value of 0.

delete:

• Use drop primary key, no attention at this time used modifyto modify.

example:

创建好以后添加主键约束：
方式一：
mysql> alter table stu modify id int primary key;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> desc stu;
+------------+-------------+------+-----+---------+-------+
| Field      | Type        | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| id         | int(11)     | NO   | PRI | NULL    |       |
| name       | varchar(20) | NO   |     | NULL    |       |
| age        | int(11)     | YES  |     | NULL    |       |
| score      | double(4,2) | YES  |     | NULL    |       |
| address_id | int(11)     | YES  |     | NULL    |       |
+------------+-------------+------+-----+---------+-------+
方式二：
mysql> alter table stu add primary key (id);
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> desc stu;
+------------+-------------+------+-----+---------+-------+
| Field      | Type        | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| id         | int(11)     | NO   | PRI | 0       |       |
| name       | varchar(20) | NO   |     | NULL    |       |
| age        | int(11)     | YES  |     | NULL    |       |
| score      | double(4,2) | YES  |     | NULL    |       |
| address_id | int(11)     | YES  |     | NULL    |       |
+------------+-------------+------+-----+---------+-------+
5 rows in set (0.01 sec)
这两种方式一个有默认值，一个没有默认值。

创建时添加主键约束：
mysql> create table stu(
    ->          id int primary key,
    ->          name varchar(20) not null,
    ->          age int,
    ->          score double(4,2),
    ->          address_id int
    -> );
Query OK, 0 rows affected (0.02 sec)
mysql> desc stu;
+------------+-------------+------+-----+---------+-------+
| Field      | Type        | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| id         | int(11)     | NO   | PRI | NULL    |       |
| name       | varchar(20) | NO   |     | NULL    |       |
| age        | int(11)     | YES  |     | NULL    |       |
| score      | double(4,2) | YES  |     | NULL    |       |
| address_id | int(11)     | YES  |     | NULL    |       |
+------------+-------------+------+-----+---------+-------+
5 rows in set (0.01 sec)

Foreign key constraint

create:

• When you create a table of creation: constraint FK_stu_address foreign key (address_id) references address(id). Where: constraintis the key constraint FK_stu_addressis the name of the foreign key, addresss_idforeign key column name, addressis the primary table name, according to their actual situation changes, constraint FK_stu_addressthis can not write.
• After you've created to add: add constraint FK_stu_address foreign key (address_id) references address(id).

delete:

• alter table stu drop foreign key FK_stu_address。

Cascade: when modifying the primary table, modify the corresponding data will follow from the table, the line followed by the direct use.

• on update cascadeCascading update
• on delete cascadeCascading delete
• Use small example:

mysql> alter table stu add constraint FK_stu_address foreign key (address_id)  REFERENCES  address(id) on update cascade;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

example:

创建以后添加外键：
mysql> alter table stu add constraint FK_stu_address foreign key (address_id)  REFERENCES  address(id);
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

创建时添加外键：
mysql> create table stu(
    ->          id int,
    ->          name varchar(20),
    ->          age int,
    ->          score double(4,2),
    ->          address_id int,
    ->          primary key(id),
    ->          constraint FK_stu_address # 这一行可以不写
    ->          foreign key(address_id)   # 外键列名
    ->          REFERENCES  address(id)   # 对应主表中的东西
    -> );
Query OK, 0 rows affected (0.02 sec)

删除外键：
mysql> alter table stu drop foreign key FK_stu_address;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0