Finding all records without associated ones

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I have a teams table and a players table and I’m wanting to find all teams that do not have players associated with them. The players table is associated via a team_id column. I’m using Ruby on Rails for this so I have a Team and a Player model.

Answer

It may perform better doing a left join:

SELECT
teams.*
FROM teams
LEFT JOIN players ON (teams.id = players.team_id)
WHERE
players.team_id IS NULL

Or using ARel (thanks to JasonKing’s comment):

Team.includes(:players).where('players.team_id IS NULL')

Attribution
Source : Link , Question Author : Ryan Bigg , Answer Author : Ryan Bigg

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