Primary Key vs. Foreign Key in Relational Databases

Primary Key vs. Foreign Key

In a relational database, keys help create structure by identifying data and defining relationships between tables. Two types of keys are primary keys and foreign keys.

Definition

Primary keys are columns in a table that uniquely identify the information in rows or tuples. Each table has just one primary key, which can have several attributes and is used to identify information within the table.

Foreign keys are columns that reference the primary key of another table, creating a link between them. Unlike primary keys, tables can have more than one foreign key, and they can reference more than one table.

Purpose

Primary keys ensure unique row identification in a table, making it easier to find and access specific information. They are a type of unique key that cannot have null values, and they are used to identify unique entries in a table.

Foreign keys, on the other hand, are used to establish a relationship between two tables by referencing the primary key or a unique key of another table. They ensure the integrity of data by preventing orphaned records, which is a record that has no connection to any other record in the database.

Uniqueness

Primary keys must be unique within a table and cannot contain null values, ensuring each row has a unique identifier. This uniqueness is enforced by the database management system.

Foreign keys can have duplicate values, but they must reference a primary key or unique key in another table. This link ensures the referenced table is consistent with the parent table.

Constraints

Primary keys have constraints that prevent duplicate values and enforce uniqueness. They cannot contain null values, which means each row must have a unique primary key.

Foreign keys also have constraints, such as referential integrity, which ensures that the data in the child table is consistent with the parent table's primary key. They can contain null values, which allows for more flexibility in the database.

Relationship between Tables

Parent tables, which are referenced by other tables, use primary keys. These primary keys are then used as foreign keys in child tables, ensuring a link between the two tables.

In summary, primary keys and foreign keys are essential in relational databases. Primary keys provide unique identification for rows within tables, while foreign keys create relationships between tables by referencing primary keys. Both keys play a crucial role in maintaining data integrity and structure in a database.