Database systems are useful for data storage and retrieval, but can also summarize your data in more concise forms. Summaries are useful when you want the overall picture, not the details. They’re more readily understood than a long list of records. They enable you to answer questions such as “How many?” or “What is the total?” or “What is the range of values?” If you run a business, you may want to know how many customers you have in each state, or how much sales volume you generate each month.
The preceding examples include two common summary types: counting summaries and content summaries. The first (the number of customer records per state) is a counting summary. The content of each record is important only for purposes of placing it into the proper group or category for counting. Such summaries are essentially histograms, where you sort items into a set of bins and count the number of items in each bin. The second example (sales volume per month) is a content summary, in which sales totals are based on sales values in order records.
Another summary type produces neither counts nor sums, but simply a list of unique values. This is useful if you care which values are present rather than how many of each there are. To determine the states in which you have customers, you need a list of the distinct state names contained in the records, not a list consisting of the state value from every record.
The summary types available to you depend ...