Primary Keys: Data Identity

Concepts: dmPrimaryKeys

A primary key is a column (or set of columns) that uniquely identifies each row. It is the minimum contract every table must fulfill: no two rows can represent the same entity. The primary key is not just a constraint. It is the mechanism by which every other operation in the relational model is made safe. Joins reference it. Foreign keys point to it. Deduplication logic anchors on it. If you choose it poorly, the damage ripples through every downstream query. What Makes a Good Primary Key Composite Primary Keys A composite PK uses two or more columns together to identify a row. They are the natural fit for junction tables in many-to-many relationships. An enrollment table linking students to courses has a composite PK of (student_id, course_id). The combination is unique even though neith