COUNT(*) Includes Null Values?

Question

MSDN documentation states:

COUNT(*) returns the number of items in a group. This includes NULL values and duplicates.

How can you have a null value in a group? Can anyone explain the point they're trying to make?

Answer 1

If you have this table

Table1:

 Field1    Field2    Field3  ---------------------------    1         1         1   NULL      NULL      NULL    2         2        NULL    1         3         1 

Then

 SELECT COUNT(*), COUNT(Field1), COUNT(Field2), COUNT(DISTINCT Field3)  FROM Table1 

Output Is:

 COUNT(*) = 4; -- count all rows, even null/duplicates   -- count only rows without null values on that field  COUNT(Field1) = COUNT(Field2) = 3   COUNT(Field3) = 2   COUNT(DISTINCT Field3) = 1 -- Ignore duplicates