SQL JOIN與AVG

Question

兩個表我試圖連接兩個表：

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 

我得到的回覆是有多個得分同一首歌的副本，我希望得分是平均的。

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 

我試過用：

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 

但是，平均所有得分，而不僅僅是得分每一個人首歌曲的

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

Answer 1

您需要GROUP BY所有字段要保留：

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，你可能只是這樣做：

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

Answer 2

使用 「組由」 songs.id

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

Answer 3

使用此 選擇a.id，a.song，一.artist，avg（b.score）作爲歌曲評分a內部加入評分b對a.id = b.id 分組a.id，a.artist，a.song

Answer 4

您需要GROUP BY並加入數據向左（JOIN LEFT）

試試這個：

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