MySQL subquery required help

i am a beginner of MYSQL please help me.

i have a two table images and rate.

i want out put is imgID Rate.

image id as p.k in images table.

image is is foriegn key in rate table.

rate table sample data is

ImgID   Rating
1       5
1       7
1       8
2       8
3       2
4       3
1       4

image table sample data is

ImgID Comment rate
1     good    7
2     bad     8
3     ok      8
4     fine    8
5     bad     7

out 开发者_高级运维put i required is

ImgId avg(rate from rate table when compared img id)

for example

ImgIf rate
-----------
1       6
2       8
3       2
4       3

i required subquery of this output.

thanks in advance

You don't need a subquery, you need an aggregation.

SELECT  ImgID,
        AVG(Rate) Rate
FROM    RateTable
GROUP BY ImgID

If you want to include all images regardless of whether there is a rating:

SELECT  i.ImgID,
        AVG(r.Rate) Rate
FROM    ImageTable i
        LEFT JOIN
                RateTable r
                ON i.ImgID = r.ImgID
GROUP BY i.ImgID