查找没有关联记录的所有记录

执行左联接可能会更好:

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

或者使用AREL(感谢Jasonking的评论):

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

像这样:

select * from teams
where id not in (select distinct team_id from players)

你可以用一个 NOT EXISTS 情况如下:

SELECT *
FROM teams
WHERE NOT EXISTS (SELECT 1 FROM players WHERE players.team_id = teams.team_id)

通常,您应该能够从父级到子级执行外部联接,并检查子级中不允许为空的字段中是否有空值。这通常比“不在”或“不存在”更快。这可能不适用于所有数据库。