Understanding Basics of Foreign Keys in MySQL?

The foreign key can be used to create a relationship between tables. The foreign key relationship can be one to one or one to many. A foreign key matches another field of another table.

• One to One relationship − One record from one table will be linked to one record in another table.

• One to Many relationships − One record will be linked to multiple records in another table.

The following is an example. First, we will create a table. The CREATE command is used to create a table.

mysql> create table tblF
    - > (
    - > id int ,
    - > FirstName varchar(100),
    - > FK_PK int
    - > );
Query OK, 0 rows affected (0.57 sec)

To create a second table.

mysql> create table tblP
    - > (
    - > FK_PK int,
    - > LastName varchar(100),
    - > primary key(FK_PK)
    - > );
Query OK, 0 rows affected (0.94 sec)

The following is the syntax to create a foreign key.

mysql> ALTER table tblF add constraint ConstFK foreign key(FK_PK) references tblP(FK_PK);
Query OK, 0 rows affected (2.17 sec)
Records: 0  Duplicates: 0  Warnings: 0

Check if foreign key has been created with the help of DESC command.

mysql> DESC tblF;

The following is the output.

+-----------+--------------+------+-----+---------+-------+
| Field     | Type         | Null | Key | Default | Extra |
+-----------+--------------+------+-----+---------+-------+
| id        | int(11)      | YES  |     | NULL    |       |
| FirstName | varchar(100) | YES  |     | NULL    |       |
| FK_PK     | int(11)      | YES  | MUL | NULL    |       |
+-----------+--------------+------+-----+---------+-------+
3 rows in set (0.05 sec)