Nothing really, unless you specify a field in a table or an expression within parantheses instead of constant values or *
Let me give you a detailed answer. Count will give you non-null record number of given field. Say you have a table named A
select 1 from A
select 0 from A
select * from A
will all return same number of records, that is the number of rows in table A. Still the output is different. If there are 3 records in table. With X and Y as field names
select 1 from A will give you
1
1
1
select 0 from A will give you
0
0
0
select * from A will give you ( assume two columns X and Y is in the table )
X Y
-- --
value1 value1
value2 (null)
value3 (null)
So, all three queries return the same number. Unless you use
select count(Y) from A
since there is only one non-null value you will get 1 as output