What is referential integrity in the context of a relational database?

Referential integrity is a fundamental concept in relational databases that ensures relationships between tables remain consistent. Specifically, it mandates that every foreign key in a table must reference a valid primary key in another table. This relationship is essential for maintaining the integrity of the data across the database. By enforcing this constraint, the database system prevents orphaned records, which are records that reference non-existent entries in related tables. This helps ensure that the data is coherent and reliable, as every entity instance is linked to a valid counterpart.

The other options, while related to database design, do not accurately define referential integrity. For instance, the requirement for tables to be in third normal form addresses the elimination of redundancy and dependency in database structure but is not directly related to the preservation of relationships between tables. Similarly, maintaining unique values in primary keys pertains to ensuring each record is distinct, which is a different aspect of database integrity. Lastly, a principle for limiting data duplication relates more to normalization processes than to the specific requirement of foreign key references. Thus, the first option embodies the essence of referential integrity within relational database management.