Relational databases manage relationships between tables using keys and constraints. Each table has a primary key that uniquely identifies its records. Relationships are established by linking these primary keys to foreign keys in other tables. There are three main relationship types: one-to-one, one-to-many, and many-to-many. For example, in an e-commerce system, a customers table might have a primary key customer_id, which is referenced as a foreign key in an orders table to link each order to a customer (a one-to-many relationship). Many-to-many relationships, like students enrolling in courses, require a junction table (e.g., enrollments) with foreign keys pointing to both primary tables.
Foreign key constraints enforce referential integrity, ensuring relationships remain valid. When a foreign key is defined, the database prevents actions that would break the link, such as deleting a customer with existing orders unless cascading rules are set. For instance, using ON DELETE CASCADE would automatically remove all related orders if a customer is deleted. Indexes on foreign keys improve query performance when joining tables. For example, an index on customer_id in the orders table speeds up queries fetching a customer’s orders. Constraints and indexes work together to maintain data consistency while optimizing access patterns.
Developers must balance referential integrity with flexibility. Overusing constraints can complicate schema changes, while underusing them risks inconsistent data. For example, a products table linked to inventory might use constraints to prevent deleting a product with remaining stock. Queries often use JOIN operations to retrieve related data: SELECT * FROM customers JOIN orders ON customers.customer_id = orders.customer_id combines customer and order data. Properly designed relationships simplify complex queries and ensure data reliability, making the database structure intuitive and maintainable.