Beginning SQL

Executing the SQL returns seven distinct values. The following Try It Out puts your knowledge of counting
and grouping results to the test.
Try It Out Counting and Grouping Results
The film club’s chairperson wants to know which film categories are most popular among club members.
The list must be in order from the most popular category to the least popular category.
1. Execute the following SQL to obtain the grouped results:
SELECT Category, COUNT(FavCategory.CategoryId) AS Popularity
FROM FavCategory INNER JOIN Category
ON FavCategory.CategoryId = Category.CategoryId
GROUP BY Category
ORDER BY Popularity DESC;
Note that the preceding code won’t work in MS Access unless you change the ORDER BY clause
as shown here:
ORDER BY COUNT(FavCategory.CategoryId) DESC;
How It Works
This SQL statement is a little trickier than what you’ve seen so far. Perhaps the hardest part with SQL is
not the actual code but rather working out how to get the information you want, determining where it’s
coming from, and then translating that into SQL. If you find that the SQL necessary to answer a particular
question is a little bit tricky, it’s best to start simple and build the code up. For example, a good start
in answering the film club chairperson’s query could be simply to select all the CategoryIds in the
FavCategory table:
SELECT FavCategory.CategoryId
FROM FavCategory;
After selecting the categories, you can count the categories:
SELECT COUNT(FavCategory.CategoryId)
FROM FavCategory;
This query returns 21, which tells you nothing except that there are 21 rows in the table. However, you
want to know how many rows there are in each category, so you need to group the results by CategoryId:
SELECT COUNT(FavCategory.CategoryId)
FROM FavCategory
GROUP BY FavCategory.CategoryId;
Executing this code gives a count for each category, but it contains only the CategoryId, so you need to
join the FavCategory and Category tables together to get the names of the category groups:
SELECT Category, COUNT(FavCategory.CategoryId) AS Popularity
FROM FavCategory INNER JOIN Category
ON FavCategory.CategoryId = Category.CategoryId
GROUP BY Category;
Grouping and Aggregating Data
195