Chapter 6. Summarizing and Grouping Data

The preceding chapter described scalar functions, which operate on individual row values. This chapter introduces SQL's aggregate functions, or set functions, which operate on a group of values to produce a single, summarizing value. You apply an aggregate to a set of rows, which can be:

  • All the rows in a table

  • Only those rows specified by a WHERE clause

  • Those rows created by a GROUP BY clause

A GROUP BY clause, which groups rows, often is used with a HAVING clause, which filters groups. No matter how many rows the input set contains, an aggregate function returns a single statistic: a sum, minimum, or average, for example.

The main difference between queries with and without aggregate functions is that nonaggregate ...

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