Foreign key

Definition

A foreign key is a field (or collection of fields) in one table that uniquely identifies a row of another table, creating a relationship between the two tables. It establishes a link between the data in the two tables, ensuring referential integrity by restricting the values that can be entered into the foreign key field to only those that exist in the corresponding primary key field of the referenced table.

5 Must Know Facts For Your Next Test

1. Foreign keys can consist of one or multiple fields, allowing for complex relationships between tables.
2. When a foreign key is defined, it can enforce rules on what data can be entered into the table, helping to prevent errors and maintain data consistency.
3. In many relational databases, attempting to delete or update a record that is referenced by a foreign key will result in an error unless specific cascading actions are defined.
4. Foreign keys play a crucial role in normalizing database design, reducing data redundancy by allowing related data to be stored in separate tables.
5. Using foreign keys enhances data integrity and facilitates easier retrieval of related information through relational queries.

Review Questions

• How does a foreign key establish relationships between tables, and why is this important for database design?
  • A foreign key creates a direct link between two tables by referencing the primary key of another table. This relationship is crucial for database design because it helps maintain referential integrity, ensuring that relationships between different data entities are preserved. By doing so, it allows for more organized data storage and reduces redundancy, making it easier to manage and query related information.
• Discuss how referential integrity is maintained through the use of foreign keys in relational databases.
  • Referential integrity is maintained through foreign keys by ensuring that any value entered into a foreign key field must correspond to an existing primary key value in the referenced table. This rule prevents orphaned records and inconsistencies within the database. If an attempt is made to add or modify data in such a way that breaks this rule, the database system will reject the change, thereby preserving the logical consistency of relationships between tables.
• Evaluate the impact of foreign keys on data retrieval and query performance within relational databases.
  • Foreign keys significantly enhance data retrieval and query performance by enabling efficient joins between tables. When queries utilize foreign keys, they can quickly access related records without having to duplicate data across tables. This not only speeds up query execution but also reduces storage requirements and improves overall database efficiency. However, excessive use of foreign keys can complicate data manipulation and potentially slow down performance if not managed properly.

Related terms

primary key: A primary key is a unique identifier for a record in a database table, ensuring that no two records can have the same value in this field.

referential integrity: Referential integrity is a database concept that ensures relationships between tables remain consistent, meaning that foreign keys must always point to valid entries in another table.

join: A join is an operation that combines rows from two or more tables based on a related column, often using foreign keys to establish connections.