Within the relational model, what best describes a key with respect to a table (relation)? Choose the statement that correctly characterizes a key's role.
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Amust always be composed of two or more columns.
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Bcan only be one column.
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Cidentifies a row.
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Didentifies a column.
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Eis only used for sorting rows.
Answer
Correct Answer: identifies a row.
Explanation
Introduction / Context:A key is central to ensuring entity integrity in a relational table. Understanding what a key actually does—beyond its physical representation—helps avoid design mistakes.
Given Data / Assumptions:
- A key may be single-attribute or multi-attribute (composite).
- The table conforms to relational rules (no duplicate rows under a candidate/primary key).
- We distinguish a key’s purpose from its implementation details.
Concept / Approach:The purpose of a key is to uniquely identify each row (tuple). Whether this key is one column or multiple columns is incidental; the defining property is uniqueness and minimality (for candidate keys).
Step-by-Step Solution:
Focus on the function, not structure → identification of a row is the function of a key.Composite vs. single-attribute keys both can serve as identifiers.Therefore, the accurate, general statement is “identifies a row.”Verification / Alternative check:Relational design guidelines define a key as a unique identifier for tuples.
Why Other Options Are Wrong:Must be two or more columns / can only be one column: Both are false—keys can be either. Identifies a column: Keys are constraints over columns; they don’t identify columns. Used only for sorting: Sorting is unrelated; keys provide uniqueness and referential anchors.
Common Pitfalls:Believing all keys must be composite or single—design drives the choice.
Final Answer:identifies a row.