Question at position 21 Relational databases establish the relationships between entities by means of common fields included in a file called a(n):relation.entity.relationship.association.单项选择题
A
relation.
B
entity.
C
relationship.
D
association.

Question

Question at position 21 Relational databases establish the relationships between entities by means of common fields included in a file called a(n):relation.entity.relationship.association.

BlackTom AI · Accepted Answer

A relation (table) is the file that contains named fields (columns) and records (rows), so common fields in two relations establish relationships between entities.

1. Relevant knowledge points and concepts
- Relational database: data stored in tables; each table is a relation.
- Relation/table: a set of tuples (rows) described by attributes (columns/fields).
- Primary key and foreign key: a primary key uniquely identifies rows in one relation; a foreign key is a field in another relation that refers to that primary key, linking the two relations.
- Entity-Relationship (ER) modeling terms: "entity" names a real-world object type; "relationship" or "association" describes how entities relate, but these are conceptual, not the physical file.

2. Step-by-step reasoning
- The question uses the word "file" in a DBMS sense; in relational terminology a file corresponds to a table or relation.
- Relationships between entities in a relational DB are implemented physically by including a common field (a foreign key) in one table that references the primary key of another table.
- Therefore the file that contains those fields is a relation (table).

3. Intermediate steps / examples
- Example: Relation Customers has CustomerID (primary key). Relation Orders has CustomerID (foreign key). The common field CustomerID in the Orders relation establishes the relationship between Customers and Orders.
- Mapping: entity (Customer) -> relation Customers; entity (Order) -> relation Orders; relationship implemented by the foreign key field present in the Orders relation.

4. Why the correct answer is correct
- "Relation." is the term for the actual file/table storing fields; relational databases use relations to implement links via common fields (keys). Thus "relation." precisely matches the physical structure described.

5. Why the other options are incorrect
- "Entity." is a conceptual object type, not the file containing fields.
- "Relationship." describes how entities relate conceptually, not the physical file storing fields.
- "Association." is a generic term similar to relationship; it is not the name of the file/table in relational theory.

6. Conclusion
The correct answer is "relation." which matches the provided answer.

Question at position 21 Relational databases establish the relationships between entities by means of common fields included in a file called a(n):relation.entity.relationship.association.单项选择题

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