What are Aggregate Functions in SQL Server? This article helps you to explore various Aggregate Functions in SQL Server.

What are Aggregate Functions?
I can give different definitions.

• Aggregate functions are built in sql server functions.
• Aggregate functions are applied to sets of records rather than to a single record.
• Aggregate functions performs a computation on a set of values rather than on a single value.
• Aggregate functions uses to summarize data.
• Aggregate functions perform a calculation on a set of values and return a single value.

Getting Started
The information in multiple records are processed in a particular manner and then displayed in a single record answer.
Aggregate functions are often used in conjuction with GROUP BY clause.
Aggregate functions cannot be nested. The expression cannot be a subquery.

The list of built in Aggregate functions are:
AVG, CHECKSUM, CHECKSUM_AGG, COUNT, COUNT_BIG, GROUPING, MAX, MIN, SUM, STDEV, STDEVP, VAR, VARP.

AVG in SQL Server
AVG returns the average of the values in expression. The expression must contain numeric values. Null values are ignored.

The syntax: AVG ([ ALL | DISTINCT ] <expression>)
select orderid, avg(UnitPrice) UnitPrice from dbo.[Order Details] group by orderid; 

CHECKSUM in SQL Server
This is a basic hash algorithm usually used to detect changes or consistency in data.

"A digit representing the sum of the correct digits in a piece of stored or transmitted digital data, against which later comparisons can be made to detect errors in the data."

The syntax: CHECKSUM(<expression>, [ ... n] | *)

SELECT CHECKSUM(orderid, UnitPrice, quantity), orderid, UnitPrice, quantity FROM dbo.[Order Details] WHERE orderid = 10248 

CHECKSUM_AGG in SQL Server
The same as CHECKSUM, but the primary difference is that CHECKSUM is oriented around rows, whereas CHECKSUM_AGG is oriented around columns.

The syntax: CHECKSUM( [ALL | DISTINCT] <expression> )
SELECT CHECKSUM_AGG(CAST(UnitPrice AS int)) FROM dbo.[Order Details] 
update dbo.[Order Details] set UnitPrice = 15 
where orderid = 10248 and ProductID = 11 
SELECT CHECKSUM_AGG(CAST(UnitPrice AS int)) FROM dbo.[Order Details] 

COUNT in SQL Server
Returns the number of items in expression. The data type returned is of type int.

The syntax: COUNT( [ALL | DISTINCT] <expression> | * )
select COUNT(*), AVG(UnitPrice) from dbo.[Order Details] 

COUNT_BIG in SQL Server
Returns the number of items in a group. The data type returned is of type bigint.

The syntax: COUNT( [ALL | DISTINCT] <expression> | * )
select COUNT_BIG(*), AVG(UnitPrice) from dbo.[Order Details] 

GROUPING in SQL Server
MSDN : Is an aggregate function that causes an additional column to be output with a value of 1 when the row is added by either the CUBE or ROLLUP operator, or 0 when the row is not the result of CUBE or ROLLUP.

Function adds an extra column to the output of a SELECT statement.

The syntax: GROUPING(<column_name> )
select orderid, sum(UnitPrice) UnitPrice, GROUPING(orderid) 'orderid' 
from dbo.[Order Details] WHERE orderid = 10248 
GROUP BY orderid WITH cube 

MAX in SQL Server
Returns the maximum value from expression. Max ignores any NULL values.

The syntax: MAX( [ALL | DISTINCT] <expression> )
select MAX(QUANTITY) from dbo.[Order Details] 

MIN in SQL Server
Returns the smallest value from expression. Min ignores any NULL values.

The syntax: MIN( [ALL | DISTINCT] <expression> )
select MIN(QUANTITY) from dbo.[Order Details] 

SUM in SQL Server
Returns the total of all values in expression. Sum ignores any NULL values.

The syntax: SUM( [ALL | DISTINCT] <expression> )
select SUM(QUANTITY) from dbo.[Order Details] 

STDEV in SQL Server
Returns the standard deviation of all values in expression. Stdev ignores any NULL values.

The syntax: STDEV( <expression> )
select STDEV(QUANTITY) from dbo.[Order Details]

STDEVP in SQL Server
Returns the standard deviation for the population of all values in expression. Stdevp ignores any NULL values.

The syntax: STDEVP( <expression> )
select STDEVP(QUANTITY) from dbo.[Order Details] 

VAR in SQL Server
Returns the variance of all values in expression. Var ignores any NULL values.

The syntax: VAR( <expression> )
select VAR(QUANTITY) from dbo.[Order Details] 

VARP in SQL Server
Returns the variance for the population of all values in expression. Varp ignores any NULL values.
The syntax: VARP( <expression> )
select VARP(QUANTITY) from dbo.[Order Details]