Using GROUP BY with aggregate functions

A GROUP BY clause almost always appears in statements that include aggregate functions, in which case the aggregate produces a value for each group. These values are called vector aggregates. (A scalar aggregate is a single value produced by an aggregate function without a GROUP BY clause.)

Example

The following query lists the average price of each kind of product:

SELECT Name, AVG( UnitPrice ) AS Price
   FROM Products
   GROUP BY Name;

Name	Price
Tee Shirt	12.333333333
Baseball Cap	9.5
Visor	7
Sweatshirt	24
...	...

The vector aggregates produced by SELECT statements with aggregates and a GROUP BY appear as columns in each row of the results. By contrast, the scalar aggregates produced by queries with aggregates and no GROUP BY also appear as columns, but with only one row. For example:

SELECT AVG( UnitPrice )
   FROM Products;

AVG(Products.UnitPrice)
13.3

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