How to add constraints in mysql?

In the MYSQL database, you can perform some operations on various items of the table when creating a table, such as adding primary key constraints or non-null constraints; you can also add constraints and delete constraints after creating the table. The following article will give you a detailed understanding, I hope it will be helpful to you.

What are constraints?

Constraints are actually restrictions on the data in the table; the purpose is to ensure that the records in the table are complete and valid.

Commonly used constraints are:

1, non-null constraint (not null)

2, unique constraint (unique)

3, primary key constraint ( primary key)

4. Foreign key constraint(foreign key)

5. Check constraint (currently not supported by MySQL, supported by Oracle)

##mysql Methods to add and delete constraints:

1. Add constraints when creating the table

create table table_name(
列名1  数据类型 (int) primary key auto_increment,
列名2 数据类型  not null,
列名3 数据类型   unique,
列名4 数据类型  default '值',
constraint  索引名 foreign key(外键列)  references 主键表(主键列)
on delete cascade | on delete set null
)

2. Add constraints after the table creation is completed And delete constraints

1), non-null constraints

Add non-null constraints

alter  table table_name modify 列名 数据类型  not null

Delete non-null constraints

alter table table_name modify 列名 数据类型 null

2), unique Constraint

Add unique constraint

alter table table_name add unique 约束名（字段）

Delete unique constraint

alter table table_name drop key 约束名

3), primary key constraint

Add primary key constraint

alter table  table_name add primary key (字段)

Delete primary key constraints

alter table table_name drop primary key

4), foreign key constraints

Add foreign key constraints

alter table table_name add constraint 约束名 foreign key(外键列)

Delete foreign key constraints

alter table table_name drop foreign key 约束名

5), automatic growth Constraints

Add automatic growth constraints

alter table table_name  modify 列名 int  auto_increment

Delete automatic growth constraints

alter table table_name modify 列名 int

Recommended related video tutorials: "

MySQL Tutorial"

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