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count aggregate function [Oracle SQL]

count (*)
count (expression)
count (all expression)
count (distinct expression)

count (expression) over (analytic-clause)
If expression in count(expression) evaluates to null, it is not counted: 
create table count_null (
  a number,
  b number
);

insert into count_null values (    1,    5);
insert into count_null values ( null,    7);
insert into count_null values ( null, null);
insert into count_null values (    8,    2);

select count(a) count_a_not_null,
       count(b) count_b_not_null,
       count(*) count_all
  from count_null;

COUNT_A_NOT_NULL COUNT_B_NOT_NULL  COUNT_ALL
---------------- ---------------- ----------
               2                3          4