How to Set foreign key constraints and the role of foreign keys in MySQL

How to Set foreign key constraints and the role of foreign keys in MySQL

1. The foreign key has two main functions:

One is to allow the database to ensure data integrity and consistency through foreign keys.

One is to increase the readability of the erimage.

2. Foreign key Configuration

1) create a master table with the following code:

# Create a table student and add various constraints

Create table student

(

Id int primary key, # primary key constraint

Name varchar (20), # unique constraint

Age int not null, # non-empty Constraint

Sex varchar (2 ),

Address varchar (20) default 'chongqing '# default Constraint

);

Create a score table by using a foreign key. The Code is as follows:

# Creating a score table

Create table score

(

Id int primary key,

Sid int,

China int,

History int,

English int,

Constraint FK_sid foreign key (sid) references student (id) # create a link through a foreign key

);

 

There are many ways to create a foreign key. The most common format for creating a foreign key is the appearance of the constraint FK _ *** foreign key (**) references link.

Delete foreign key:

Alter table drop foreign key 'foreign key name '.

Note:

You can use the constraint foreign key name foreign key... to delete a foreign key only when defining a foreign key.

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