Beginning SQL

Chapter 6
SELECT Category, COUNT(FavCategory.CategoryId) AS Popularity
FROM FavCategory INNER JOIN Category
ON FavCategory.CategoryId = Category.CategoryId
GROUP BY Category.Category
HAVING COUNT(FavCategory.CategoryId) > 3
ORDER BY Popularity DESC;
If you’re using MS Access, you need to change the ORDER BY clause as shown below:
ORDER BY COUNT(FavCategory.CategoryId) DESC;
How It Works
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The best way to deal with queries that seem a bit tricky at first (recall the discussion of the COUNT()
function) is to break them up into smaller stages.
You know that the FavCategory table contains data on each member’s favorite film categories, so that is
your starting point; this leads you to the following SQL:
SELECT FavCategory.CategoryId, COUNT(FavCategory.CategoryId) AS Popularity
FROM FavCategory
GROUP BY FavCategory.CategoryId;
While numbers are fine for a computer, humans prefer names. In order to make the answer more readable,
you need to display the category name. To obtain the category name, you need to access the Category table,
linking the results set so far with the data in the Category table via the CategoryId field. To do this, you
need to join the tables with an inner join, resulting in the following SQL:
SELECT Category.Category, COUNT(FavCategory.CategoryId) AS Popularity
FROM FavCategory INNER JOIN Category
ON FavCategory.CategoryId = Category.CategoryId
GROUP BY Category.Category;
Next, to ensure that the results are ordered correctly (by popularity of category), add an ORDER BY
clause:
SELECT Category.Category, COUNT(FavCategory.CategoryId) AS Popularity
FROM FavCategory INNER JOIN Category
ON FavCategory.CategoryId = Category.CategoryId
GROUP BY Category.Category
ORDER BY Popularity DESC;
Remember, though, that you want only groups where four or more members chose that category as their
favorite, which is where a HAVING clause comes in. You can use a HAVING clause to restrict the final
results to just those groups with four or more records inside them by adding HAVING
COUNT(FavCategory.CategoryId) > 3:
SELECT Category, COUNT(FavCategory.CategoryId) AS Popularity
FROM FavCategory INNER JOIN Category
ON FavCategory.CategoryId = Category.CategoryId
GROUP BY Category.Category
HAVING COUNT(FavCategory.CategoryId) > 3
ORDER BY Popularity DESC;