Aggregate functions always return a single value.
The Transact-SQL language supports six aggregate functions:
AVG
MAX
MIN
SUM
COUNT
COUNT_BIG
AVG (short for average) calculates the arithmetic mean of the data values contained within a column. The column must contain numeric values.
MAX calculates the maximum, and MIN the minimum, data value of the column.
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 function is analogous to the COUNT function.
The only difference is that COUNT_BIG returns a value of the BIGINT data type.