MySQL 查询 GROUP、COUNT

Question

I have problem with MySQL GROUP AND COUNT, they just don't work in the way I thought.

Table1
ID | group_
1    1
2    1 
3    2  

Table2
ID | score
1    100
1    80  
2    50  
3    50 

OUTPUT
group_ | group_avg_score | group_num
1        70                2
2        50                1

This is my MySQL query and error outputs. Note that for group 1 avg_score should be ID1 avg_score 90 and ID2 avg_score 50, (90+50)/2=70

SELECT  A.group_, ROUND(AVG(B.score),2) as group_avg_score, COUNT(*) as group_num
FROM Table1 A, Table2 B
WHERE A.ID=B.ID
GROUP BY group_

OUTPUT
group_ | group_avg_score | group_num
1        76.67             3
2        50                1

How can I fix it?

Answer 1

From your explanation, I can see that you want an average of averages. This can be solved with a derived table (nested select in FROM clause). Calculate the "inner" average first as such:

SELECT ID, AVG(score) AS score
FROM Table2
GROUP BY ID

And then, nest that query as follows:

SELECT Table1.group_, AVG(Table2.score) AS group_avg_score, COUNT(*) AS group_num
FROM Table1
JOIN (
  SELECT ID, AVG(score) AS score
  FROM Table2
  GROUP BY ID
) Table2
ON Table1.ID = Table2.ID
GROUP BY Table1.group_

SQLFiddle