Question

What will COUNT(*) returns if all the collection has only null values???

Answer 1

Aggregate function (like Count(), Sum(), Avg()) ignore null values.
Now what about count(*) ?

In case of Count (Marks) it count number of  NON NULL rows. Count(Marks) discards those rows in which Null is present. Here Domain is Marks Column.

Same Analogy with Count(*) also. It counts every non null rows. 
let there is Student table S and with attributes :  Name, DOB, Subject & Marks.

Now Count(*) count no of  row in which atleast one column is Non NULL otherwise discard that row. If every column of a particular row is filled with NULL (which really not a good database) then count(*) also ignore that row. For Count(*) domain is all column so it will check null value in every column unlike Count(Marks) see NULL values only in Marks Column.


Count(), Count(*) both excludes those rows in which every column is NULL.

Comments

@Digvijay sir 

Isn't count(*) is an exception here?
In case where every column of a particular row is filled with NULL, count(*) will include it in counting. 

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you r right

Answer 2

In SQL,when we use the count function over an attribute of the relation and that attribute contains null values for some/all tuples of that relation than the null values will not be considered during the calculation.But if we use count(*) over some relation than it will consider the null values as well.So count(*) will return count of all the tuples in that relation.

Answer 3

count(*) will return count of all the non-null  tuples in the relation & ignore the count for tuples with all null values.

Answer 4

this might help you

https://stackoverflow.com/questions/40679208/count-includes-null-values