primaryKey

A primaryKey, commonly referred to as a primary key in relational databases, is a column or set of columns in a table that uniquely identifies each row. It serves as the principal means of locating records and supports entity integrity by preventing duplicate or missing identifiers.

Key properties include uniqueness and non-nullability. Each row must have a unique value for the primary key, and null values are not permitted. Typically there is only one primary key per table, but it can be composed of multiple columns to form a composite primary key.

Implementation and usage: In SQL, a primary key is declared with a PRIMARY KEY constraint on the chosen column(s). This often creates a unique index to optimize lookups. Many database systems also support auto-incrementing primary keys, which automatically generate new identifiers for new rows.

Design considerations: Some designs favor surrogate keys, which are artificial numeric identifiers used solely to identify rows, to avoid coupling meaning to the identifier. Others use natural keys based on real-world attributes. Primary keys are referenced by foreign keys in related tables to enforce referential integrity and model relationships between tables. Proper selection of a primary key influences data integrity, query performance, and maintainability across the database schema.