Foreign keys can be used to create relationships between tables. Foreign key relationships can be one-to-one or one-to-many. A foreign key matches another field in another table.
One-to-one relationship - A record in one table will be linked to a record in another table.
One-to-many relationship - One record will be linked to multiple records in another table.
Below is an example. First, we will create a table. 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)
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 for creating foreign keys.
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
Use the DESC command to check whether the foreign key has been created.
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)
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