SQL JOIN与AVG

Question

两个表我试图连接两个表：

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 

我得到的回复是有多个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 

我试过用：

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，而不仅仅是得分每一个人首歌曲的

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，你可能只是这样做：

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 “组由” 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 选择a.id，a.song，一.artist，avg（b.score）作为歌曲评分a内部加入评分b对a.id = b.id 分组a.id，a.artist，a.song

Answer 4

您需要GROUP BY并加入数据向左（JOIN LEFT）

试试这个：

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