这是执行MySQL查询的最佳/有效方式

Question

1st approach :-

SELECT 
    clipComment.commentId,
    clipComment.commentedBy,
    clipComment.clipId AS commentTypeId,
    'clip' AS commentType,
    clipComment.commentDescription,
    clipComment.commentCreatedDateTime,
    clipComment.commentModifiedDateTime,
    clipComment.commentLikeCount,
    userProfile.userName,
    userProfile.firstName,
    userProfile.LastName,
    userProfile.profilePicUrl,
    userProfile.themeForeground,
    userProfile.themeBackground,
    IF(derCommentLike.commentId = clipComment.commentId,
        1,
        0) likedByMe
FROM
    clipComment
LEFT JOIN
    (SELECT 
        *
    FROM
        clipCommentLikes
    WHERE
        commentLikedBy = 16) derCommentLike 
ON 
    derCommentLike.commentId = clipComment.commentId
LEFT JOIN
    userProfile 
ON 
    userProfile.userId = clipComment.commentedBy
WHERE
    clipComment.clipId = 141

2nd approach :-

SELECT
    clipComment.commentId,
    clipComment.commentedBy,
    clipComment.clipId AS commentTypeId,
    'clip' AS commentType,
    clipComment.commentDescription,
    clipComment.commentCreatedDateTime,
    clipComment.commentModifiedDateTime,
    clipComment.commentLikeCount,
    userProfile.userName,
    userProfile.firstName,
    userProfile.LastName,
    userProfile.profilePicUrl,
    userProfile.themeForeground,
    userProfile.themeBackground,
    IF( derCommentLike.commentId = clipComment.commentId , 1 , 0 ) AS likedByMe
FROM
    (SELECT
        *
     FROM
        clipCommentLikes
     WHERE
        commentLikedBy = 16) derCommentLike
    RIGHT OUTER JOIN clipComment
     ON derCommentLike.commentId = clipComment.commentId
    RIGHT OUTER JOIN userProfile
     ON clipComment.commentedBy = userProfile.userId
WHERE
    clipComment.clipId = 141

both query returns same result, but just want to know which approach should i follow & which one is more efficient to follow. record set will contain millions of record, so i want to use best way. or i am doing work then please correct me. thank you in advance.

explain statement 1st approach

explain statement 2nd approach

explain statement 1st approach

Answer 1

    IF(derCommentLike.commentId = clipComment.commentId, 1, 0) likedByMe
...
LEFT JOIN
    (SELECT  *
        FROM clipCommentLikes
        WHERE commentLikedBy = 16
    ) derCommentLike 
   ON    derCommentLike.commentId = clipComment.commentId

-->

   ( EXISTS SELECT * FROM clipCommentLikes
         WHERE commentId = clipComment.commentId
   ) AS  likedByMe

Explanation:

• JOIN ( SELECT ... ) has no index to make it efficient
• LEFT JOIN ( ... ) begs for evaluating the subquery after the left table, thereby begging for the subquery to be evaluated repeatedly.
• SELECT * (in your subquery) is gathering lots of stuff that is not used. ( SELECT * in EXISTS does not mean to fetch everything; the * is just a place holder.)
• EXISTS evaluates to 1 (true) or 0 (false), which seems to be what you want.