### 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