mysql如何选择在另一列中具有相同值且在另一表的列中具有相同值的字段？

Question

This is part of a homework to be perfectly honest, not of such importance, but still. So, lets suppose we have the following tables:

        Song                                      Singer_prod

composer     title                            singer      sing_title
   A          a1                                S1            b1
   A          a2                                S3            c3
   B          b1                                S3            c1
   C          c1                                S1            a1
   C          c2                                S1            a2
   C          c3                                S3            c2 
                                                S2            a1

Now, the question is to select those composers, whose every title has been sang by only one singer. For instance, composer B has only one title, b1 , sang only by S1 .

Also, composer C has written c1, c2,c3 , all sang only by S3 . Those two composers should show up in the results.

In contrast, from composer's A songs, a2 was sang only by S1 but a1 was sang both by S1 and by S2 , so A shouldn't show up in the results.

I've tried countless combinations of nested SELECTS, GROUP BYs and WHEREs, with no luck so far. The only thing i'm pretty sure is that a count(singer)=1 should be needed somewhere, so far fruitlessly used though.

Any help would be greatly appreciated!

Answer 1

SELECT Composer, Singer FROM Song x
INNER JOIN Singer_prod y on x.title = y.sing_title
GROUP BY Composer, Singer
HAVING COUNT(Distinct Singer) = 1