SQL One-to-One Relationships

What & Why

A one-to-one relationship means each row in table A relates to at most one row in table B, and vice versa. This often shows up when a table is deliberately split in two — perhaps for security reasons, or to avoid a table with an unwieldy number of columns.

See How It Works

WITH latest_session AS (
  SELECT
    user_id,
    MAX(started_at) AS latest_session_at
  FROM growth.sessions
  GROUP BY user_id
)
SELECT
  u.id AS user_id,
  u.plan,
  s.latest_session_at
FROM growth.users u
LEFT JOIN latest_session s ON s.user_id = u.id
ORDER BY user_id;