Primary Key

A primary key is the column (or set of columns) whose value uniquely identifies each row in a table — like a student ID number where no two students share the same one. It is how you point to one specific record without ambiguity.

Keys are the glue of relational data: a good primary key must be unique and never missing. A related idea is the foreign key, a column in one table that refers to another table's primary key (an order's customer_id pointing to the customers table) — this is exactly what joins use to connect tables.