Foreign key

Definition

A foreign key is a column or a set of columns in a relational database table that creates a link between the data in two tables. This key serves as a reference to the primary key of another table, establishing a relationship that ensures data integrity and enforces referential constraints, which helps maintain consistency across related data sets.

5 Must Know Facts For Your Next Test

1. Foreign keys can be created using the `FOREIGN KEY` constraint in SQL when defining or altering a table.
2. A foreign key can reference the primary key of another table, creating a one-to-many relationship between the tables.
3. If a record in the parent table (the table containing the primary key) is deleted or updated, it can affect records in the child table (the table containing the foreign key), depending on the specified actions for cascading updates or deletes.
4. Foreign keys help enforce data integrity by ensuring that every value in the foreign key column matches an existing value in the referenced primary key column.
5. Using foreign keys allows for better organization of data and supports complex queries that require data from multiple related tables.

Review Questions

• How do foreign keys contribute to data integrity in relational databases?
  • Foreign keys help maintain data integrity by ensuring that relationships between tables are valid. When a foreign key is defined, it requires that any value in that column must match an existing value in the corresponding primary key column of the related table. This enforces referential integrity and prevents orphaned records, ensuring all data across related tables remains consistent.
• What happens to records in a child table when a corresponding record in the parent table is deleted or updated, and how can this behavior be controlled?
  • When a corresponding record in the parent table is deleted or updated, it can affect records in the child table based on the cascading rules defined with the foreign key constraint. If cascading updates or deletes are enabled, changes will automatically reflect in the child table. Alternatively, if these rules are not set, attempts to delete or update parent records may fail if there are still dependent records in the child table, thereby maintaining referential integrity.
• Analyze how using foreign keys and joins together can enhance database querying and data retrieval.
  • Using foreign keys alongside joins significantly enhances database querying by allowing for efficient retrieval of related data across multiple tables. Foreign keys define relationships between tables, which enable joins to combine records based on these connections. This means that complex queries can be executed more easily, providing users with comprehensive datasets without redundancy. This capability not only improves data retrieval efficiency but also ensures that queries return consistent and meaningful results reflecting the relationships established by foreign keys.

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 that column or set of columns.

referential integrity: Referential integrity is a property of a database that ensures relationships between tables remain consistent, meaning that any foreign key must correspond to an existing primary key in another table.

join: A join is an SQL operation that combines rows from two or more tables based on a related column, often involving foreign keys to establish the connection between those tables.