Keys: what makes a row that row

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Concept · step 1 of 11

What a key is

A key is a column, or a set of columns, whose value identifies one row and no other. The primary key is the one a table is known by.

A key never repeats. no two customers share an id.
A key is never missing. a row with no id could not be told apart from anything.
Postgres enforces both for a `PRIMARY KEY`, and keeps a unique index behind it. the lightning bolt on the board.

CREATE TABLE customers (
  id   BIGINT PRIMARY KEY,
  name TEXT   NOT NULL
);

Ada is customer 1 and Linus is customer 2, and nothing can ever make that ambiguous. Every other table that needs to point at a customer will point at this id, which is why it has to be this strict.

PostgreSQL docs: CREATE TABLE, PRIMARY KEY and UNIQUE ↗

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