What is a foreign key?

A foreign key is a database constraint that creates a relationship between two tables based on the values in one or more columns in each table. It is used to enforce referential integrity, which ensures that the data in one table is consistent with the data in another table.

In a foreign key relationship, one table is the parent or referenced table, and the other table is the child or referencing table. The foreign key in the child table references the primary key in the parent table, which means that the values in the foreign key column must match the values in the primary key column.

Foreign key constraints are used to maintain the relationships between tables and ensure that any changes made to the data in one table are reflected in the data in the other table. For example, if a record is deleted from the parent table, any related records in the child table should also be deleted or updated to reflect the change.

Foreign keys are commonly used in relational databases to implement one-to-many or many-to-many relationships between tables. For example, in a database of orders and customers, the customer ID in the orders table would be a foreign key that references the customer ID in the customers table. This would allow the database to associate each order with the customer who placed it.