In chapter 11, we summarized all the data in a column of a table. The result was a single value. In this chapter, we divide the rows of the table into groups, which are nonoverlapping sets of rows. Each group is summarized separately, resulting in a summary value for each of the groups.
At our discretion, we can either summarize a column into a single value or divide it into 100 pieces and summarize each piece. This gives us control over the level of detail we want to see.