如何更改PostgreSQL表并使列唯一？

ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE (column_name);

ALTER TABLE users ADD CONSTRAINT unique_email UNIQUE (email);

DELETE FROM table_name WHERE column_name IN (
  SELECT column_name FROM (
    SELECT column_name, ROW_NUMBER() OVER (PARTITION BY column_name ORDER BY id) AS row_num
    FROM table_name
  ) AS subquery
  WHERE row_num > 1
);

DELETE FROM users WHERE email IN (
  SELECT email FROM (
    SELECT email, ROW_NUMBER() OVER (PARTITION BY email ORDER BY id) AS row_num
    FROM users
  ) AS subquery
  WHERE row_num > 1
);

ALTER TABLE table_name ALTER COLUMN column_name SET NOT NULL;

ALTER TABLE users ALTER COLUMN email SET NOT NULL;