UNIQUE: no two alike, or no two among some

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No two rows share it

A UNIQUE constraint says no two rows hold the same value. Like a key, it can span several columns.

On one column. email TEXT UNIQUE.
On several, written after the last column. UNIQUE (team_id, user_id), where the pair must not repeat but each half may.
Enforced by a unique index Postgres builds for it, the lightning bolt on the board..

CREATE TABLE memberships (
  id      BIGINT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
  team_id BIGINT NOT NULL,
  user_id BIGINT NOT NULL,
  UNIQUE (team_id, user_id)
);

User 1 is in teams 1 and 2, and team 1 has users 1 and 2: every half repeats, no pair does. The table also keeps a surrogate id as its primary key; a table can have one primary key and any number of UNIQUE constraints.

PostgreSQL docs: Constraints, unique constraints ↗

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