Difference between count(col_nm) and count(*)

Question

Hi i tried the following
create table chknull (a )
as select 1 from dual union all
select 2 from dual union all
select null from dual And i selected the values which are not null
select count(*) from chknull where a is nullReturns me 1 record
But
select count(a) from chknull where a is nullReturns me 0 record
Why it returns like above;
Could you please give me the hint in this
Thanks

Answer 1

Count(<expression>) where expression is a column, a literal or a calculation, counts all non-null occurences of the expression. Since your column a is null, the count of non-null occurences of the expression is zero. Count(*) is a special case of count(<expression>) where the expression evaluates to a row. So, as long as the row exists count(*) will count it.
Consider:
SQL> WITH t AS (
  2     SELECT 1 id, 'A' name FROM dual UNION ALL
  3     SELECT 2, 'B'  FROM dual UNION ALL
  4     SELECT 3, null  FROM dual)
  5  SELECT count(*) star, count('Banana') literal, count(name) col,
  6         count(case when mod(id, 2) = 0 then 1 end) calulation
  7  FROM t;
      STAR    LITERAL        COL CALULATION
         3          3          2          1John