Joe Celko's SQL for Smarties, 3rd Edition

23.5. Average Deviation

If you have a version of SQL with an absolute value function, ABS(), you can also compute the average deviation following this pattern:

BEGIN
SELECT AVG(x) INTO :average FROM Samples;
SELECT SUM(ABS(x - :average)) / COUNT(x) AS AverDeviation
  FROM Samples;
END;

This is a measure of how far data values drift away from the average, without any consideration of the direction of the drift.