Each table in a relational database has one field designated as its primary:

Question

BlackTom AI · Accepted Answer

A primary key uniquely identifies each record (tuple) in a table (relation), so the correct completion is "key."

1. Relevant knowledge points and concepts
- Relation/table: a set of tuples (rows) representing instances of an entity.
- Attribute/field/column: a named property of the relation describing one piece of data per tuple.
- Record/tuple: a single row containing values for each attribute.
- Primary key: an attribute or combination of attributes chosen to uniquely identify each tuple; enforces entity integrity.
- Candidate key, composite key, surrogate vs. natural key, and foreign keys (which reference primary keys) are related concepts.

2. Step-by-step reasoning
- The phrase asks for the term used when one field in a table is designated as its primary X.
- The only database concept that fits "primary X" and denotes uniqueness and identification of rows is "primary key."
- Primary keys ensure no two rows share the same primary key value (uniqueness) and that the key is not NULL (entity integrity).
- Therefore the missing word is "key."

3. Calculations or intermediate steps
- No numerical calculations are needed; the reasoning is conceptual and based on definitions and integrity rules of relational databases.

4. Why the correct answer is correct
- "Primary key" is the standard term in relational database theory: a primary key is a chosen candidate key that uniquely identifies tuples in a relation. It is commonly described as “one field designated as the primary key” (or a composite of fields). The primary key enforces uniqueness and non-null constraints, which matches the intent of the sentence.

5. Why the other options are incorrect
- "Attribute": redundant with "field" — you wouldn’t say “primary attribute” in this context.
- "Entity": an entity is the real-world object or row concept, not a field in a table.
- "Relation": is another name for table, not a field within it.
- "Record": refers to a row/tuple, not a designated field.

6. Conclusion
The correct answer is "key." This matches the provided answer.

Each table in a relational database has one field designated as its primary: 单项选择题

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