In relational database terminology, a subset of the database that contains virtual data derived from base tables but is not explicitly stored is called what?

Introduction / Context:
Relational databases allow users to define virtual tables that present data in convenient ways without duplicating storage. These virtual tables are defined by queries over base tables and are recalculated when accessed. Understanding the concept and name of this virtual subset of the database is important for designing secure and flexible schemas. This question asks for the correct term used to describe such a subset.

Given Data / Assumptions:

We are using a relational database that supports the concept of views.
The subset of the database is defined in terms of queries over existing base tables.
The data for this subset is not stored separately as physical rows; it is generated when the object is referenced.
The question contrasts this object with tuples, relations, and keys.
We are using standard relational terminology.

Concept / Approach:
A view is a named query stored in the database that defines a virtual table. When you query the view, the database engine executes the underlying query and returns rows as if they were stored in a real table. However, the data is not usually stored separately, unless the view is materialized in special cases. In relational terminology, a tuple is a single row, a relation is a base table or a view in the abstract sense, and a key is a set of attributes used to identify rows. Therefore, the specific object described in the question is most accurately called a view.

Step-by-Step Solution:
Step 1: Identify that the subset of the database is virtual and derived from existing tables via a definition rather than being stored as independent data. Step 2: Recall that a view is defined by a CREATE VIEW statement that stores the query definition, not the result set itself, in the system catalog. Step 3: Recognize that when the view is queried, the result set is computed from the base tables according to the stored definition, which matches the idea of virtual data described in the question. Step 4: Select view as the correct term, and confirm that tuple, relation, and key do not match the description.

Verification / Alternative check:
Consider a base table Employees and a view that shows only active employees with selected columns. CREATE VIEW ActiveEmployees AS SELECT name, department FROM Employees WHERE status = 'Active'. When you query ActiveEmployees, the database computes the result from Employees each time. No separate storage of ActiveEmployees rows is required. This example illustrates that the subset is virtual and is correctly called a view.

Why Other Options Are Wrong:
A tuple is a single row of a relation, not a whole subset or virtual table.
A relation is a generic term for a table but does not specifically denote a virtual subset defined by a query. The question emphasises derived, non stored data, which points to views rather than base relations.
A key is a set of attributes used to identify tuples, not a subset of the database.

Common Pitfalls:
One pitfall is to confuse views with physical tables and assume that views duplicate data, which can lead to incorrect assumptions about storage and performance. Another is to avoid using views because of performance concerns without properly testing, even though views can simplify security and encapsulate complex joins. Understanding that a view is a virtual, not physically stored, subset of the database helps designers use them appropriately.

Final Answer:
Such a virtual subset of the database is called a view.