Practical SQL Handbook, The: Using SQL Variants, Fourth Edition

The GROUP BY Clause

The GROUP BY clause is intimately connected to aggregates. In fact, GROUP BY doesn't really have much use without aggregate functions: it divides a table into sets of rows, and aggregate functions produce summary values for each set. These values are called vector aggregates. (A scalar aggregate is a single value produced by an aggregate function as discussed in the previous chapter.) Figure 6.1 shows examples of scalar and vector aggregate queries.

Figure 6.1. Scalar and Vector Aggregates