SQL Server count function

SQL Server Count() functions could be used as sql count command to count aggregate values.
Count () function has optional argument and expression. To find the general count of values we could use different forms count or supply different argument as described below and it will return the same value.

USE AdventureWorks;
GO
 
--COUNT ALL FirstName
SELECT count(ALL FirstName ) AS CountOfPerson
FROM Person.Person;
 
--COUNT FirstName
SELECT count(FirstName ) AS CountOfPerson
FROM Person.Person;
 
--COUNT *
SELECT count(* ) AS CountOfPerson
FROM Person.Person;
GO

We may also need to query a distinct FirstName values in our table and to do that we could use a DISTINCT argument inside the Count() function.

--COUNT DISTINCT FirstName
SELECT count(DISTINCT FirstName ) AS CountOfPerson
FROM Person.Person;

Read more:
http://technet.microsoft.com/en-us/library/ms175997.aspx