Foreign Key Mapping is the process of establishing a link between two records. Suppose I have a User and I make a table called users that has some specific ID that should be a primary key.
An additional table named ‘vehicles’ has a listing of vehicles with descriptions such as the vehicle model, brand, color, and so on. The thing that comes to mind is that I have to establish a relationship with the vehicles’ owners. I am going to add a new column named user_id which works as a foreign key to the user’s table in the vehicles table.
SQL Query Example:
ALTER TABLE `vehicles` ADD CONSTRAINT `user_vehicles` FOREIGN KEY (`user_id`) REFERENCES `users`(`id`) ON DELETE CASCADE ON UPDATE CASCADE;
One-to-one mapping occurs when records of a primary table relate to a single record of a secondary table. For example, a relationship between the User table and the UserDetails table.
Another case occurs when a record is linked to multiple records. For example, users may have multiple vehicles, multiple phone numbers, multiple documents, etc.
Whenever a record of a first table can reference multiple records of a second table and similarly a second table record can reference multiple records of a first table, it refers to many-to-many mapping. As an example, a product could have multiple tags and each tag may relate to multiple products.
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