在SQL查询中联接4个表

如果 所有表 都具有与Friends表的链接，则当前查询仅列出结果。我相信您正在寻找类似这样的东西

SELECt followingFROM   friendsWHERe  follower='id'UNIOn ALL SELECt followingFROM   family WHERe  follower='id'UNIOn ALL SELECt followingFROM   followingWHERe  follower='id'UNIOn ALL SELECt followingFROM   acquaintances WHERe  follower='id'

或更好地阅读并且更容易以一些性能为代价进行调整

SELECt following FROM   (        SELECt following, follower FROM friends        UNIOn ALL SELECt following, follower FROM family         UNIOn ALL SELECt following, follower FROM following        UNIOn ALL SELECt following, follower FROM acquaintances        ) AS fWHERe  follower='id'

联盟

UNIOn用于将来自多个SELECT语句的结果合并为一个结果集。