A primary key in SQL is a column or set of columns that uniquely identifies each row in a table. Its primary purpose is to enforce entity integrity, ensuring no duplicate or null values exist for the designated column(s). For example, in a users table, a column like user_id could serve as the primary key, guaranteeing each user has a unique identifier. Primary keys are fundamental to relational database design because they enable precise referencing of records in other tables through foreign keys, forming the basis for table relationships. Without a primary key, managing data consistency and establishing meaningful connections between tables becomes difficult.
Primary keys must adhere to two core rules: uniqueness and non-nullability. Each value in the primary key column(s) must be unique, and no value can be missing. A primary key can be a single column (e.g., an auto-incremented integer like order_id in an orders table) or a composite key combining multiple columns. For instance, in an order_items table, both order_id and product_id might form a composite primary key to uniquely identify each item within an order. Composite keys are useful when no single column uniquely identifies a row, but they should be used sparingly to avoid complexity. Choosing the right primary key—whether natural (e.g., a government-issued ID number) or surrogate (e.g., an auto-generated number)—depends on the data’s stability and uniqueness guarantees.
Beyond identification, primary keys optimize database performance. Most database systems automatically create a clustered index on the primary key, physically ordering the table’s data to speed up queries. This makes primary key lookups highly efficient. However, poorly chosen primary keys (e.g., wide composite keys or frequently updated columns) can degrade performance. For example, using a UUID as a primary key might introduce fragmentation in the clustered index, slowing write operations. Developers should also ensure foreign keys in related tables correctly reference the primary key to maintain referential integrity. In summary, primary keys are essential for structuring reliable, efficient databases and enabling accurate data relationships.