Database Integrity and Keys

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Questions and Answers

What does the entity integrity constraint ensure regarding primary key values?

They are optional in a relation.
They should match values in foreign keys.
They can contain NULL values.
They must be unique across all tuples. (correct)

Which statement best describes referential integrity?

It guarantees all primary keys are non-NULL.
It ensures that foreign key values must match existing primary key values in another relation. (correct)
It is concerned only with the uniqueness of foreign keys.
It allows tuples in one relation to reference non-existent tuples in another.

In the context of a foreign key, what is required of the attributes in FK?

They can only contain NULL values.
They must be a subset of the referencing relation's attributes.
They must have the same domain as the primary key attributes of the referenced relation. (correct)
They must have different domains than the primary key attributes.

What does it mean if a foreign key value is NULL in the referencing relation?

The foreign key value does not have to match a primary key value in the referenced relation. (A) Signup and view all the answers

Which of the following statements is NOT a characteristic of primary keys?

They can contain duplicate values. (B) Signup and view all the answers

What is the role of a foreign key in relation to referential integrity?

It links attributes in the referencing relation to the primary key in the referenced relation. (D) Signup and view all the answers

If a foreign key in EMPLOYEE references the Dnumber in DEPARTMENT, what must be true?

Dno can be NULL in EMPLOYEE if it has no matching Dnumber. (C) Signup and view all the answers

What happens if a primary key in the referenced relation is updated?

The foreign keys referencing that primary key must also be updated. (B) Signup and view all the answers

What does a referential integrity constraint from R1 to R2 ensure?

R1 must reference at least one primary key of R2. (A) Signup and view all the answers

In the EMPLOYEE relation, what does the Dno attribute represent?

The department number where the employee works. (A) Signup and view all the answers

Which attribute serves as a foreign key that references the EMPLOYEE relation itself?

Super_ssn (C) Signup and view all the answers

How can referential integrity constraints be visually indicated in a relational database schema?

By drawing a directed arc from the foreign key to the primary key of the referenced relation. (C) Signup and view all the answers

What requirement must be fulfilled for a foreign key to reference another relation?

The foreign key must have a matching primary key in the referenced relation. (C) Signup and view all the answers

When can a foreign key in the EMPLOYEE relation be set to NULL?

When the employee does not belong to any department. (C) Signup and view all the answers

What is a common method for specifying integrity constraints in a relational database?

Including them as part of the database definition within the DDL. (C) Signup and view all the answers

What role do referential integrity constraints play in a relational database?

They ensure data consistency across related tables. (C) Signup and view all the answers

Study Notes

Entity Integrity

Entity integrity constraint ensures no primary key value can be NULL; it is essential for identifying individual tuples in a relation.
NULL primary key values result in ambiguous identification, complicating tuple references.

Referential Integrity

Referential integrity constraint maintains consistency between tuples in two relations.
A tuple must refer to an existing tuple in another relation for the referential integrity to hold.
Example: In the EMPLOYEE relation, the Dno attribute must match existing Dnumber values in the DEPARTMENT relation.

Foreign Keys

A foreign key in relation schema R1 references relation R2 based on specific rules:
- Attributes in the foreign key (FK) share the same domain as primary key attributes (PK) in R2.
- A value of FK in relation R1 must either correspond to a PK value in R2 or be NULL.
R1 is referred to as the referencing relation, while R2 is the referenced relation.

Relationships in Databases

Referential integrity constraints often arise from relationships between different entity schemas in a database.
Example: In EMPLOYEE, Dno indicates the department an employee works for, which is a foreign key pointing to DEPARTMENT.

Self-Referencing Foreign Keys

A foreign key can refer to its own relation, as seen with the Super_ssn attribute in EMPLOYEE, indicating an employee's supervisor.
Example: John Smith references Franklin Wong as his supervisor within the EMPLOYEE relation.

Diagrammatic Representation

Referential integrity constraints can be visually represented with directed arcs showing relationships between foreign keys and their referenced tables.
Arrowheads point to primary keys of the referenced relations for clarity.

Database Schema and Integrity Constraints

All integrity constraints need to be defined within the relational database schema for enforcement by the DBMS.
Data Definition Language (DDL) provisions allow for the specification of various constraints to ensure rules are automatically enforced.

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Description

This quiz covers essential concepts related to entity integrity, referential integrity, and the role of foreign keys in relational databases. Understanding these principles is crucial for maintaining the integrity and reliability of database systems. Test your knowledge and grasp of these foundational elements.