Beginning SQL

Appendix A
380
You might receive slightly different results after the decimal point due to rounding errors by database
systems.
As with all of the more complex queries, start simple and build up. The first job is identifying which
tables contain the data needed to answer the question. With this example, the Films table contains the
price and film category information. However, you want to display the category name rather than just
an ID, and name data comes from the Category table. Use a FROM statement to join the two tables, linked
by the CategoryId, which is the primary key in the Category table and a foreign key in the Films table:
FROM Films
INNER JOIN Category
ON Films.CategoryId = Category.CategoryId
WHERE AvailableOnDVD = ‘Y’;
Add a WHERE clause that includes only records where the AvailableOnDVD field is Y, as there’s no point
counting films that are not available on DVD.
The question requires that the results be grouped into categories and that the results should return the
category name, the number of DVDs in the category, and the total price for the DVDs in each category
group. Therefore, you need to add a GROUP BY clause, grouping by Category:
FROM Films
INNER JOIN Category
ON Films.CategoryId = Category.CategoryId
WHERE AvailableOnDVD = ‘Y’
GROUP BY Category;
Finally, add the columns and expressions that make up the final results: the Category column, the count of
the number of DVDs per group, and the total price of the DVDs in that group including a 10% sales tax:
SELECT Category, COUNT(DVDPrice), SUM(DVDPrice * 1.1)
FROM Films
INNER JOIN Category
ON Films.CategoryId = Category.CategoryId
WHERE AvailableOnDVD = ‘Y’
GROUP BY Category;
The second part of the question asks you to limit the results to just those groups where the number of
DVDs is just one. To do this, add a HAVING clause with a condition that allows only groups with one
record in them to be included in the final results:
SELECT Category, COUNT(DVDPrice), SUM(DVDPrice * 1.1)
FROM Films
INNER JOIN Category
ON Films.CategoryId = Category.CategoryId
WHERE AvailableOnDVD = ‘Y’
GROUP BY Category
HAVING COUNT(DVDPrice) = 1;
The final SQL provides the results in the following table: