Final answer:
When designing or normalizing relations in a database, it is generally recommended that each relation should have only one theme. This helps to improve the efficiency and maintainability of the database.
Step-by-step explanation:
When designing or normalizing relations in a database, it is generally recommended that each relation should have only one theme. This means that each relation should represent a single concept or entity, and not mix multiple unrelated concepts together. Having a single theme for each relation helps to improve the efficiency and maintainability of the database.