AVG « Aggregate Functions « SQL Server / T-SQL Tutorial

9.2.1.AVG calculates the average for selected records of numeric data in a column or the average for DISTINCT (unique) values of the selected records.
9.2.2.Using DISTINCT in Aggregate Functions
9.2.3.A summary query that uses the COUNT, AVG, and SUM functions
9.2.4.adds the WHERE statement to calculate the average
9.2.5.Selecting titles that sell more than the average.
9.2.6.CAST(AVG(dec1) AS dec(5,2))