Aggregate Functions in SQL SERVER

What are Aggregate functions? Explain Aggregate functions in SQL SERVER 2008 with example.

Aggregate functions are applied to a group of data values from a column. Aggregate functions always return a single value.

SQL SERVER 2008 / Transact-SQL supports following aggregate functions:

AVG: Calculates the arithmetic mean (average) of the data values contained within a column. The column must contain numeric values.

MAX and MIN: Calculate the maximum and minimum data value of the column, respectively. The column can contain numeric, string, and date/time values.

SUM: Calculates the total of all data values in a column. The column must contain numeric values.

COUNT: Calculates the number of (non-null) data values in a column. The only aggregate function not being applied to columns is COUNT(*). This function returns the number of rows (whether or not particular columns have NULL values).

COUNT_BIG: New and Analogous to COUNT, the only difference being that COUNT_BIG returns a value of the BIGINT data type.

Aggregate function Example:
SELECT ProjectName, SUM(budget) TotalBudget FROM Project_Tbl GROUP BY ProjectName;