Special behaviour of Aggregate or Set operation functions(SQL SERVER)

If you are calculating the average of employe salaries from employe table,
The aggregate functions (example AVG()) : Null values are ignored.

Note: 1. Null value is eliminated by an aggregate or other SET operations.
2. For Count,Count_Big(For number of items in a group -- returns type of bigint.), they don’t ignore NULL or duplicates. Remaining aggregate functions ignores NULL values.

Take care about these aggregation while analyzing the values....