Postgres 13

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CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
  username VARCHAR(100) NOT NULL UNIQUE
);

CREATE TABLE groups (
  id SERIAL PRIMARY KEY,
  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
);

CREATE TABLE groups_users (
  group_id INT NOT NULL,
  user_id INT NOT NULL,
  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
  CONSTRAINT fk_group
  FOREIGN KEY(group_id)
  REFERENCES groups(id),
  CONSTRAINT fk_user
  FOREIGN KEY(user_id)
  REFERENCES users(id)
);

INSERT INTO users (username)
VALUES ('bob'), ('john'), ('steve');

INSERT INTO groups DEFAULT VALUES;
INSERT INTO groups DEFAULT VALUES;
INSERT INTO groups DEFAULT VALUES;

INSERT INTO groups_users (group_id, user_id)
VALUES (1, 1),
    (1, 2),
    (2, 2),
    (2, 3),
    (3, 1),
    (3, 2),
    (3, 3);
3 rows affected

1 rows affected

1 rows affected

1 rows affected

7 rows affected

SELECT
    group_id,
    ARRAY_AGG(user_id)
FROM
    groups_users
GROUP BY group_id


group_id
array_agg


3
{1,2,3}

2
{2,3}

1
{1,2}


SELECT
    group_id
FROM
    groups_users
GROUP BY group_id
HAVING ARRAY_AGG(user_id) = ARRAY[1,2]


group_id


1