COUNT_BIG (Transact-SQL)
SQL Server 2008 R2
Returns the number of items in a group. COUNT_BIG works like the COUNT function. The only difference between the two functions is their return values. COUNT_BIG always returns a bigint data type value. COUNT always returns an int data type value. May be followed by the OVER Clause (Transact-SQL).
COUNT_BIG(*) returns the number of items in a group. This includes NULL values and duplicates.
COUNT_BIG(ALL expression) evaluates expression for each row in a group and returns the number of nonnull values.
COUNT_BIG(DISTINCT expression) evaluates expression for each row in a group and returns the number of unique, nonnull values.