Beginning SQL

Executing the final SQL provides the final results, shown in the following table:
Category Popularity
War 4
Notice also that the ORDER BY clause must come last in the SELECT statement, otherwise an error occurs.
Summary
This chapter was all about summarizing and aggregating data, rather than getting results based on individual
records. Central to this concept was the GROUP BY statement, which does the following:
❑ Enables results to be based on groups of common data
❑ Allows the aggregation and summary of those groups when used in conjunction with SQL’s
aggregate functions such as COUNT(), SUM(), AVG(), MAX(), and MIN()
The chapter examined thoroughly each of the aggregation functions, including the following:
❑ COUNT(), which counts the number of record in the results set.
Grouping and Aggregating Data
❑ SUM(), which adds together the value of the specified column or expression for each record in
the results set. SUM() works only with numerical data types.
❑ AVG(), which finds the average value for a particular column for each record in a results set.
Like SUM(), AVG() works only with numerical data types.
❑ MAX() and MIN(), which find the lowest or highest value for a column in a results set. MAX()
and MIN() work with date, time, character, and numerical data types.
Finally, this chapter explored the HAVING clause, which filters out the result of groups using various conditions,
much like a WHERE clause does for a FROM statement. Whereas the WHERE clause filters records,
the HAVING clause filters groups found by the GROUP BY clause.
The next chapter returns to joining tables, looks at subqueries, and shows you how to create complex
queries.
Exercises
1. The film club’s chairperson decides that she’d like a list showing all the film costs broken down
by film category and detailing how many films the club would have to buy in each category.
She also wants you to factor in a 10% sales tax into the cost of each DVD. Change the results to
list only those categories where one DVD is listed.
2. Write a SQL query that finds out the highest and lowest ratings for each film category.
205