Sams Teach Yourself SQL in 10 Minutes, Second Edition

Using Aggregate Functions

It is often necessary to summarize data without actually retrieving it all, and SQL provides special functions for this purpose. Using these functions, SQL queries are often used to retrieve data for analysis and reporting purposes. Examples of this type of retrieval are

• Determining the number of rows in a table (or the number of rows that meet some condition or contain a specific value).

• Obtaining the sum of a set of rows in a table.

• Finding the highest, lowest, and average values in a table column (either for all rows or for specific rows).

In each of these examples, you want a summary of the data in a table, not the actual data itself. Therefore, returning the actual table data would be a waste of time and processing ...