8.3 Calculating a Mean8.3.1 ProblemYou want to calculate the average life of light bulbs in a sample, where the sample consists of all bulbs tested in a particular test pit. 8.3.2 SolutionComputing a mean is fairly easy, because the standard SQL function AVG produces the desired result. For example: SELECT AVG(Hours) Mean FROM BulbLife WHERE TestPit=1 Mean ----------- 1089 8.3.3 DiscussionProbably the easiest of all statistics to compute using SQL is the mean. The mean is just a simple average implemented by the standard SQL function AVG. The AVG function is a group function, which means that it operates on a group of rows. In the recipe solution, the group in question consisted of all rows for Test Pit #1. Using the GROUP BY clause, you can extend the query to report the mean bulb life for all test pits. For example: SELECT TestPit, AVG(hours) Mean FROM BulbLife GROUP BY TestPit TestPit Mean ----------- ----------- 1 1089 2 1093 Using the HAVING clause, you can implement measurement rules requiring that results only be reported for those test pits that have a specified minimum number of measurements available. For example, the following query limits the report to test pits where more than eight light bulbs have been tested: SELECT TestPit, AVG(hours) Mean FROM BulbLife GROUP BY TestPit HAVING COUNT(*) >= 8 |