Relational requirements: Which of the following is NOT a restriction for a table to qualify as a relation under the relational model?
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AThe cells of the table must contain a single value.
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BAll of the entries in any column must be of the same kind.
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CThe columns must be ordered.
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DNo two rows in a table may be identical.
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EColumn names must be unique within a table.
Answer
Correct Answer: The columns must be ordered.
Explanation
Introduction / Context:Relations have formal properties that distinguish them from arbitrary tables. These properties underpin set-based reasoning and eliminate ambiguity in queries.
Given Data / Assumptions:
- Using classical relational model assumptions.
- Atomicity of cells (1NF) is required.
- Row uniqueness is a requirement for a relation.
- Attributes have names and defined domains.
Concept / Approach:In the relational model, tuples are unordered and attributes are a set, not an ordered list. While SQL interfaces may display columns in an order, the model does not require any column ordering. Atomic values, uniform domains per attribute, unique rows, and unique attribute names are expected.
Step-by-Step Solution:
Check each property: atomic cells → required.Column entries same type/domain → required.No duplicate rows → required.Column order → NOT required in the model (order is not semantically significant).Verification / Alternative check:Relational algebra treats relations as sets of tuples over a set of attributes; sets are unordered.
Why Other Options Are Wrong:They each state valid relational restrictions (atomicity, domain consistency, row uniqueness, unique attribute names).
Common Pitfalls:Assuming the presentation order in a UI reflects a theoretical requirement; it does not.
Final Answer:The columns must be ordered.