Define a valid foreign key for relations or implement the valuer/scanner interface

A valid foreign key for relations:

A foreign key is a field in one table that refers to the primary key in another table. It is used to establish a link or relationship between two tables in a database. In order for a foreign key to be valid, it must meet the following criteria:

• The column that contains the foreign key must exist in both the referring table (foreign key table) and the referenced table (primary key table).
• The data type of the foreign key column must match the data type of the referenced primary key column.
• The values in the foreign key column must exist in the referenced primary key column.

Let’s take an example to illustrate a valid foreign key:

We have two tables, “Customers” and “Orders”. The “Orders” table has a foreign key column called “customer_id” that refers to the primary key column “id” in the “Customers” table.

    Customers table:
    +----+----------+-----+
    | id | name     | age |
    +----+----------+-----+
    | 1  | John Doe | 30  |
    +----+----------+-----+
    
    Orders table:
    +----+------------------+---------+
    | id | product          | customer_id |
    +----+------------------+-------------+
    | 1  | Laptop           | 1           |
    | 2  | Smartphone       | 1           |
    | 3  | Headphones       | 2           |
    +----+------------------+-------------+
  

In the above example, the “customer_id” column in the “Orders” table is a valid foreign key because:

• The column “customer_id” exists in both the “Orders” and “Customers” table.
• The data type of the “customer_id” column matches the data type of the “id” column in the “Customers” table (both are integers).
• The values in the “customer_id” column (1 and 2) exist in the “id” column of the “Customers” table.