What are surrogate keys in relational databases?

Surrogate keys in relational databases are artificial identifiers added to tables to uniquely identify records. These keys are not derived from the application data itself but are instead generated by the system. A common example is an auto-incrementing integer column (e.g., id in a users table) or a GUID (Globally Unique Identifier). Unlike natural keys, which are based on existing business data like email addresses or product codes, surrogate keys have no inherent meaning outside the database. Their sole purpose is to provide a stable, simple way to reference rows, especially when natural keys are unreliable or complex.

Developers use surrogate keys for several practical reasons. First, natural keys often change due to business requirements (e.g., a user’s email might be updated), which can complicate foreign key relationships and require cascading updates. Surrogate keys remain static, avoiding this problem. Second, they simplify joins between tables. For instance, linking an orders table to a customers table via a customer_id integer column is more efficient and readable than using a composite key like (first_name, last_name, phone_number). Surrogate keys also reduce the risk of errors when natural keys are long, contain special characters, or are prone to duplication. For example, using an auto-incremented product_id is safer than relying on a product name that might be reused or misspelled.

While surrogate keys are widely useful, they require careful implementation. Developers must still enforce uniqueness on natural keys where necessary—for example, adding a unique constraint on a user_email column even if a user_id surrogate key exists. Surrogate keys also add a layer of abstraction: applications might need to join tables to retrieve meaningful business data, which can slightly impact query complexity. However, these trade-offs are typically outweighed by the benefits of stability and simplicity. In practice, surrogate and natural keys often coexist, with the former handling relationships and the latter enforcing business-specific uniqueness rules.