SQL & PL/SQL

Using Aggregating Functions Like MIN,MAX,AVG,SUM on null values
the Result is Null ..But for Count it will return 0 ..Can anybody tell specific reason for this


for eg: select min(null) from dual -- > null
select max(null) from dual --> null
---
select count(null) from dual -->null or 0 ?

and now let us take this example

select count('a') from dual; --> 1
select count(null) from dual --> 0 ? it shud atleast return 1
is not considered as value in dual table ?



i dont understand why -- select count(null) from dual; is returning 0 ;