从映射表中获取一个查询中的数据

如果用户对给定用户的每个帖子都喜欢自己的帖子，请在子查询和COALESCE上使用LEFT JOIN来获取真/假（1/0）标志

    
      SELECT p.*, COALESCE(isLikedByUSer, 0)
FROM Posts p
LEFT JOIN (SELECT 1 isLikedByUser, postId, userID FROM LikeMapping) as l ON l.postId = p.postId AND l.userId = p.userId
WHERE p.userId = 100
</code>
  

这可能比其他答案更快。

    
      SELECT p.*,
       lm.postid IS NOT NULL AS isLikedByUser
    FROM Posts AS p
    LEFT JOIN ( SELECT DISTINCT postid FROM LikeMapping ) AS lm
          ON lm.postid = p.postid
</code>