sql get a unique ID then count the number of tuples relating to that ID - sql

Question

Database Structure
MovieInfo (mvID, title, rating, year, length, studio)
DirectorInfo(directorID, firstname, lastname)
MemberInfo(username, email, password)
ActorInfo(actorID, firstname, lastname, gender, birthplace)
CastInfo(mvID*, actorID*)
DirectInfo(mvID*, directorID*)
GenreInfo(mvID*, genre)
RankingInfo(username*, mvID*, score, voteDate)
Query
I need to get the director with the largest number of comedy movies. (I'm also required to use the ALL operator). My understanding is getting the list of mvid where genre = 'Comedy" and directorid:
select mvid
from genreinfo
where genre = 'Comedy'
union all
select directorid
from directorinfo
;
But then how do I count the number of movies a specific director has? And how do I get that single one with the highest count of "comedy" movies?

Answer 1

You're on the right track. I'd recommend looking at JOINs.
I've provided a step-by-step answer on how to obtain the desired results. If you just want the final query, go down to step 5 and pick the one appropriate for your DBMS.
1: Selecting all comedy movie IDs:
SELECT mvid
FROM GenreInfo
WHERE genre = 'Comedy';
2: Selecting the directorIDs of those movies
SELECT directorID
FROM DirectInfo
JOIN GenreInfo
ON DirectInfo.mvID = GenreInfo.mvID
WHERE genre = 'Comedy';
3: Selecting the director name of those directors.
SELECT firstname
FROM DirectorInfo
JOIN DirectInfo
ON DirectorInfo.directorID = DirectInfo.directorID
JOIN GenreInfo
ON DirectInfo.mvID = GenreInfo.mvID
WHERE genre = 'Comedy';
4: Grouping that query by director to get number of movies:
SELECT firstname, COUNT(*) AS NumberOfMovies
FROM DirectorInfo
JOIN DirectInfo
ON DirectorInfo.directorID = DirectInfo.directorID
JOIN GenreInfo
ON DirectInfo.mvID = GenreInfo.mvID
WHERE genre = 'Comedy'
GROUP BY DirectorInfo.directorID;
5: Sort the results and get only the first one:
SELECT firstname, COUNT(*) AS NumberOfMovies
FROM DirectorInfo
JOIN DirectInfo
ON DirectorInfo.directorID = DirectInfo.directorID
JOIN GenreInfo
ON DirectInfo.mvID = GenreInfo.mvID
WHERE genre = 'Comedy'
GROUP BY DirectorInfo.directorID
ORDER BY NumberOfMovies
LIMIT 1;
If you're using SQL server, use TOP instead:
SELECT TOP 1 firstname, COUNT(*) AS NumberOfMovies
FROM DirectorInfo
JOIN DirectInfo
ON DirectorInfo.directorID = DirectInfo.directorID
JOIN GenreInfo
ON DirectInfo.mvID = GenreInfo.mvID
WHERE genre = 'Comedy'
GROUP BY DirectorInfo.directorID
ORDER BY NumberOfMovies;

Answer 2

You can use a join and group by to get the result.
select DirectorID,COUNT(mvid)
from DirectInfo d
inner join genreinfo g
ON d.mvid=g.mvid
where genre ='Comedy'
GROUP BY DirectorID
ORDER BY COUNT(mvid)

Answer 3

This is homework? Well, right now you are selecting a list of IDs, some of them representing directors, others representing movies. You notice that this is not at all what you are supposed to do, right?
What you want is a list of directors. So you select from the DirectorInfo table. You also want information about his movies (excatly: the number of movies of a certain kind). So you must join that information from MovieInfo. Now think about what else you need to glue together to get from director to their movies. Then think about how to glue in that genre criterium.
Once you have joined it all together, then you group your results. You want one record per director (instead of ane record per director and movie), so you make a group and count within that group.
I hope this helps you solve your task. Good luck!

Answer 4

select di.directorid, count(1) as 'no_of_comedy_movies'
from DirectorInfo di inner join join DirectInfo dri
on di.directorid = dri.directorid
inner join genreinfo gi
on gi.mvid = dri.mvid
where gi.genre = 'Comedy'
group by dri.directorID
order by no_of_comedy_movies