You typically use a query to locate all the records that meet some criteria. But sometimes you are not as interested in the details of all the records as you are in summarizing them in some way. For example, you might want to know how many orders have been placed this year or the total dollar value of all orders placed. The easiest way to get this information is by creating a query that groups the necessary fields and does the math for you. To do this, you use aggregate functions in the query.
Access queries support the aggregate functions shown in the following table:
Total of the values in a field
Average of the values in a field
Number of values in a field, not counting Null ...