Why does the count() function require a parameter?

Question

The count() function counts rows, and is very often used as count(*). If I had a table with phoneNo and firstName, count(phoneNumber) returns the same number as count(*). All columns are always present, if not populated, in each row.

Answer 1

There is a subtle difference.

If you specify a field , count(fieldname) will count records which have non null values for the field. count(*) will give you total rows.