How does a foreign key constraint ensure referential integrity?

A foreign key constraint ensures referential integrity by linking two tables and restricting actions that would break this link.

In more detail, a foreign key is a column or a set of columns in a relational database table that provides a link between data in two tables. It acts as a cross-reference between tables as it references the primary key of another table, thereby establishing a link between them. This is where the concept of referential integrity comes in. Referential integrity is a property of data stating that all its references are valid and ensuring that the relationship between tables remains consistent.

When a foreign key constraint is applied, the database management system (DBMS) prevents any actions that would cause the data in the tables to become inconsistent. This means that if there is a foreign key in a table, you cannot insert a new record into the table that does not correspond to an existing record in the linked table. Similarly, if a record in the linked table is deleted, any corresponding records in the table with the foreign key would also have to be deleted to maintain referential integrity. This is known as a cascading delete.

Furthermore, if a primary key changes in the linked table, all corresponding foreign keys in the other table would also have to be updated. This is known as a cascading update. These cascading actions ensure that the relationship between the tables remains consistent, thereby maintaining referential integrity.

In essence, a foreign key constraint is a rule enforced by the DBMS to prevent the creation of inconsistent data. It ensures that the relationship between tables is always maintained, and that there are no orphan records, i.e., records that reference non-existent data. This is crucial in maintaining the accuracy and reliability of the data in a relational database.