Beginning SQL

Chapter 8
4. You should receive the following results:
Category
Thriller
War
Sci-fi
Historical
How It Works
In the first step, you simply selected all the category names from the Category table — the main table
from which the data is extracted. The subqueries come in Step 2 to filter the results.
In Step 2, begin with the rating first. Information about films’ ratings is contained in the Films table, but
all you want to know is whether there are films in a particular category that have a rating higher than 3.
You’re not interested in knowing the names of the films, so you used an EXISTS operator with a subquery
that returns a list of films rated higher than 3.
Next, you linked the outer and inner queries using the CategoryId column in the Category and Films
tables. Now, if for a particular category there are films rated higher than 3, the subquery returns rows
and EXISTS evaluates to true.
Finally, you used another subquery, nested inside the other subquery. This innermost subquery counts
how many members like a particular category; then in the WHERE clause of the outer subquery, you check
to see whether that value is 3 or more.
Using the HAVING Clause with Subqueries
252
Chapter 5 first introduced the HAVING clause, which is used to filter the groups displayed in a results set
when a GROUP BY clause has been used. For example, to get a list of cities where the average year of
birth of members in each city is greater than 1990, you could use the HAVING clause like this:
SELECT City
FROM MemberDetails
GROUP BY City
HAVING AVG(YEAR(DateOfBirth)) > 1990;
The preceding query compares against an actual value, 1990. A subquery is useful where you want to
compare against a value extracted from the database itself rather than a predetermined value. For example,
you might be asked to create a list of cities where the average year of birth is later than the average
for the membership as a whole. To do this, you could use a HAVING clause plus a subquery that finds out
the average year of birth of members:
SELECT City
FROM MemberDetails
GROUP BY City
HAVING AVG(YEAR(DateOfBirth)) >
(SELECT AVG(YEAR(DateOfBirth)) FROM MemberDetails);