Final answer:
In a relational database, every table should have a primary key as a unique identifier for each record. Other elements like indexes or triggers augment the database's functionality but are not as fundamentally required as the primary key.
Step-by-step explanation:
In an efficiently designed relational database, every table should have a primary key. A primary key is a unique identifier for each record in the table, which ensures that no two rows can have the same value. This uniqueness constraint facilitates efficient data retrieval and management. Other options like an index, trigger, and schema may be associated with a table but are not as fundamentally necessary as the primary key.
For instance, a table containing user information may have a primary key associated with the user ID. The user ID acts as a unique identifier and can be used to reference that specific user across the database or in other related tables. Primary keys can consist of one or more columns in the table, known as a composite key, but they must uniquely identify a record.