Beginning SQL

6
Grouping and Aggregating
Data
So far, results returned by SELECT queries have been a list of records, that is, specific data rather
than summaries or overviews. This chapter examines two main things: sorting data into groups
and returning the data for that group as a whole, and aggregation functions available in SQL.
Aggregation is another way of saying a summary of data. For example, aggregating data might
involve finding the average age of film club members or counting how many members live in a
particular state. What you’ve learned so far allows you to answer questions pertaining to which
film categories each member likes or what John Jones’s favorite film category is. However, by the
end of this chapter, using a combination of groups and aggregation will enable you to answer
questions such as how many members like thrillers. The difference between grouping and aggregation
is that grouping finds out information about a particular record, whereas aggregation summarizes
more than one record.
Specifically, this chapter covers the GROUP BY clause, which groups results according to the
parameters set forth in the clause. Additionally, this chapter examines the COUNT() function,
which counts records; the SUM() function, which adds the value of records together; the AVG()
function, which finds averages; and finally, the MAX() and MIN() functions, which find the lowest
and highest values in a set of records, respectively. The chapter begins by looking at grouping
results with the GROUP BY clause.
Grouping Results
This section examines the GROUP BY clause, which is used in conjunction with the SELECT statement.
It allows you to group identical data into one subset rather than listing each record. The
GROUP BY clause is at its most powerful when used with SQL’s summarizing and aggregating
functions, which are covered in the next section. The GROUP BY clause is also very useful with
subqueries, a concept examined in Chapter 7. The aim of this section is to get a handle on how
GROUP BY works; the next section shows you how to use it more effectively.