Primary Key
A primary key is a column (or set of columns) that uniquely identifies each row in a table, enforced by the database to be unique and non-null.
Every well-designed table has a primary key so each row can be referenced unambiguously. The database rejects duplicate or null values in the key, guaranteeing identity.
Keys are often surrogate (an auto-generated identity or UUID) or natural (a real-world unique value like an email). Surrogate keys are common because they're stable and compact.
Primary keys are the targets that foreign keys reference, making them the backbone of relational integrity.
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