Question

I am using SQL Server 2008 R2

I am trying to write a single query that will return only exactly what I need. I will drop in a MovieID and get back a list of ALL genres. If the movie represents a specific genre (has an associated record in the junction table), the Checked value will be 1. If not, then 0.

My result set should look like this:

GenreID    Genre    Checked
1          ABC      0
2          DEF      1
3          HIJ      0
4          KLM      1

My First table is named Genres. It looks like this:

GenreID    Genre
1          ABC
2          DEF
3          HIJ
4          KLM

My second table is named Movies. It looks like this:

MovieID   Title
1         Blah
2         Foo
3         Carpe
4         Diem

My third table is a junction table named Movies_Genres. It looks like this:

MovieID   GenreID
1         2
1         1
1         4
2         1
2         3
3         4
4         1

I would normally, do a couple of queries and a couple of loops to handle this, but I want to really just make the database do the work here. How do I tweak my query so that I can get the resultset that I need with just a single query?

Here’s the starting query:

SELECT  GenreID, 
        Genre
FROM    Genres

Thanks in advance for your help!!!

Answer 1

SELECT g.GenreID, g.Genre, Checked = CASE WHEN EXISTS
      (SELECT 1 FROM dbo.Movies_Genres AS mg
         INNER JOIN dbo.Movies AS m
         ON mg.MovieID = m.MovieID
         WHERE mg.GenreID = g.GenreID
         AND m.MovieID = @MovieID) THEN 1 ELSE 0 END
FROM dbo.Genres AS g
ORDER BY g.GenreID;

If there is a unique constraint or primary key on dbo.Movies_Genres(MovieID, GenreID) then this can be simply:

SELECT g.GenreID, g.Genre, Checked = COUNT(mg.GenreID)
    FROM dbo.Genres AS g
    LEFT OUTER JOIN dbo.Movies_Genres AS mg
    ON g.GenreID = mg.GenreID
    AND mg.MovieID = @MovieID
    GROUP BY g.GenreID, g.Genre;

…since the count for any genre can only be 0 or 1 given a single @MovieID.