Relational Database Terminology

Core terms 核心术语

Entity 实体

• Definition: An object about which data can be stored.
• In a relational database, the entity is usually represented as a table.

可以被存储数据的对象，在数据库中表现为一张表。

• Good answer pattern: “An entity is a thing/object in the real world that is represented in a database as a table.”
• Avoid: Saying “a table” without linking it to a real‑world object.

Attribute 属性

• Definition: An item of data about an entity.
• It describes a property or characteristic of that entity.

描述实体的一个属性或特征。

• Use: “A column in a table” is sometimes accepted, but must be clearly linked to an entity.

Primary Key 主键

• Definition: The unique attribute (or combination of attributes) used to identify each record/tuple in a table.

唯一标识表中每一条记录的属性或属性组合。

• Good answer pattern: “The primary key is the unique identifier for a record.”

Foreign Key 外键

• Definition: A field in one table that is the primary key in another table.
• It is used to implement relationships between tables.

外键是一个表中的字段，它是另一个表的主键，用于实现关系。

• How it is used: In a 1:M relationship, the primary key of the “one” side appears as a foreign key on the “many” side.

Candidate Key 候选键

• Definition: A field that could be a primary key but is not.
• It can be an attribute, or the smallest set of attributes, where no two tuples have the same value.

可以作为主键但未被选中的字段。

• Use: It is a possible primary key.

Tuple 元组

• Definition: A row/record in a table; one instance of an entity.

表中的一行记录，实体的一个具体实例。

Relationships 关系

Common exam instruction: When drawing E‑R diagrams or describing relationships, always state both the cardinality and which keys are used. Example: “The relationship between SHIP and CONTAINER is one‑to‑many; the primary key ShipID in SHIP is linked to the foreign key ShipID in CONTAINER.” (Reference: 9618_w25_qp_12_4_a)

Referential Integrity 参照完整性

• Definition: Ensures that data is consistent across related tables.

确保相关联的表之间数据保持一致。

• Key points (choose the phrasing that matches the question):
• Every foreign key must match an existing primary key in the referenced table.
• A record cannot be added to a referencing table if its foreign key does not exist in the parent table.
• Cascading update/delete: If a primary key is changed or deleted, the change is automatically propagated to all matching foreign keys, preventing orphaned records.
• Prevents deletion of a parent record if child records still reference it (unless cascade is specified).

Good answer pattern (3‑mark explain question):

“Referential integrity maintains consistency between tables. It ensures that every foreign key has a corresponding primary key. It also supports cascading update/delete, so when a primary key is modified or deleted, all related foreign key values are updated or deleted automatically.”

Example from evidence (9618_w25_qp_11_2_d): In the WORKEXPERIENCE database, CompanyID in PLACEMENT depends on CompanyID in COMPANY; referential integrity would stop a placement from being created for a non‑existent company, and could cascade the deletion of a student to related placements.

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Evidence for all definitions is drawn directly from the listed mark schemes. Where multiple acceptable wordings exist, the most mark‑secure phrasing has been chosen.