How to Use MySQL count

How many ways to use count? count(*): returns a count of the number of all rows (including NULL). count(1): 1 evaludates to non-NULL for every row, so it returns the same results as count(*) count(col_name): returns a count of the number of the rows that col_name is not NULL count(col_name) VS count(*)？ count(*): returns a count of the number of all rows (including NULL). count(col_name): returns a count of the number of the rows that col_name is not NULL count(*) is recommended even though count(pk) can return the same result.