Beginning SQL

Appendix A
Your answer may differ, as there’s more than one way to answer this question, but that doesn’t matter as
long as the results are the same. The right outer join obtains all rows from the Category table, regardless
of whether there is a matching CategoryId in the FavCategory table. If there is no matching CategoryId
in the FavCategory table, then it means that no member has chosen that category as their favorite. When
this happens, FavCategory.CategoryId has a NULL value, so all you need to do is add a WHERE clause
that specifies that FavCategory.CategoryId must be NULL, and you ensure that only those categories
not picked as a favorite are included in the results. The final results are as follows:
Category
Film Noir
Chapter 8
Exercise 1 Solution
384
Obtaining the CategoryId, name, and total cost of films in a particular category is fairly easy. The difficult
part is obtaining the count of how many members put that category down as one of their favorites.
For this, you should use a subquery in the SELECT statement’s column listing.
SELECT Category.Category,
SUM(DVDPrice) AS “Total Cost For Category”,
(SELECT COUNT(*) FROM FavCategory
WHERE FavCategory.CategoryId =
Category.CategoryId) AS “Members Favorite Category”
FROM Films INNER JOIN Category
ON Films.CategoryId = Category.CategoryId
WHERE Films.AvailableOnDVD = ‘Y’
GROUP BY Category.CategoryId, Category.Category;
In the preceding query, the following subquery obtains the count of how many members listed that category
as their favorite:
(SELECT COUNT(*) FROM FavCategory
WHERE FavCategory.CategoryId = Category.CategoryId)
The subquery is joined to the outer query on the CategoryId columns in the FavCategory and Category
tables.
Notice that because the subquery references the CategoryId from the Category table in the main query,
you must include it in the GROUP BY clause.