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

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Definition of a Valid Foreign Key for Relations

A foreign key is a field or combination of fields in a database table that refers to the primary key of another table. It establishes a relationship between two tables by referencing the primary key of the related table.

To define a valid foreign key for relations, you need to ensure the following:

  • The foreign key column(s) must exist in the referencing table.
  • The foreign key column(s) must have the same data type(s) as the primary key column(s) of the referenced table.
  • The foreign key column(s) should enforce referential integrity, meaning they should only contain values that exist in the referenced table’s primary key column(s).

Let’s consider an example to further illustrate this:

    
CREATE TABLE Customers (
  customer_id INT PRIMARY KEY,
  customer_name VARCHAR(100),
  country_id INT,
  CONSTRAINT fk_country FOREIGN KEY (country_id) REFERENCES Countries (country_id)
);

CREATE TABLE Countries (
  country_id INT PRIMARY KEY,
  country_name VARCHAR(100)
);

In this example, the “Customers” table has a foreign key named “country_id” that references the primary key “country_id” of the “Countries” table. The foreign key column “country_id” in the “Customers” table has the same data type (INT) as the primary key column “country_id” of the “Countries” table.

The foreign key constraint “fk_country” ensures that the values in the “country_id” column of the “Customers” table only contain valid country IDs that exist in the “Countries” table.

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