COUNT(*):
Will count all the rows in the specified table
COUNT(Column Name)
Returns the number of rows which have a value (NULL values will not be counted)
--Or--
Some combinations of Count function in SQL Server and their description.
COUNT(*) returns the number of items in a group. This includes NULL values and duplicates.
COUNT(1) returns the number of items in a group. This includes NULL values and duplicates.
COUNT(ALL expression) evaluates expression for each row in a group and returns the number of nonnull values.
COUNT(DISTINCT expression) evaluates expression for each row in a group and returns the number of unique, nonnull values.
For return values greater than 2^31-1, COUNT produces an error. Use COUNT_BIG instead.
This is applicable on below versions of SQL Server
SQL Server 2005
SQL Server 2008 R2
SQL Server 2012
SQL Server 2014
