SQL用AVG联接两个表 [英] SQL JOIN two tables with AVG

问题描述

我正试图联接两个表:

songs
id | song | artist
---|------|-------
1  | foo  | bar
2  | fuu  | bor
3  | fyy  | bir

score
id | score
---|------
1  | 2
2  | 4
3  | 8
2  | 6
3  | 2

使用此SQL命令:

SELECT songs.id, songs.song, songs.artist, score.score FROM songs LEFT JOIN score ON score.id=songs.id ORDER BY songs.id, score DESC

我得到的是同一首歌曲的重复，并具有多个乐谱，我希望将乐谱平均.

What I get back is duplicates of the same song with multiple scores, I would like the score to be averaged.

result
id | song | artist | score
---|------|--------|-------
1  | foo  | bar    | 2
2  | fuu  | bor    | 4
2  | fuu  | bor    | 6
3  | fyy  | bir    | 8
3  | fyy  | bir    | 2

我尝试使用:

SELECT songs.id, songs.song, songs.artist, ROUND(AVG(score.score),1) AS 'score' FROM songs INNER JOIN score ON score.id=songs.id ORDER BY score DESC

但这平均所有分数，而不仅仅是每首歌曲的分数

But that averages all scores, not just the score of each individual song

result
id | song | artist | score
---|------|--------|-------
1  | foo  | bar    | 4.4

推荐答案

您需要对要保留的所有字段进行GROUP BY:

You need to GROUP BY all the fields you want to retain:

SELECT songs.id, songs.song, songs.artist, 
    AVG(score.score * 1.0) AS AvgScore
FROM songs 
    LEFT JOIN score 
        ON score.id=songs.id 
GROUP BY songs.id, songs.song, songs.artist
ORDER BY songs.id, score DESC

或者，您可以执行以下操作:

Alternatively, you could just do this:

SELECT songs.id, songs.song, songs.artist, 
    (SELECT AVG(Score) FROM score WHERE score.id = songs.id) AS AvgScore)
FROM songs 

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