e.g. count and distinct aggregate functions

examples; count (1) - count(*) - count(column_name) - count(distinct (column_name))

e.g. count aggregate function
SQL>
SELECT
    COUNT(*)
FROM
    employees;

  COUNT(*)
----------
       107

SQL>
SELECT
    COUNT(1)
FROM
    employees;

  COUNT(*)
----------
       107

SQL>
SELECT
    COUNT(commission_pct)
FROM
    employees;

COUNT(COMMISSION_PCT)
---------------------
                   35

Info: COUNT(1) and COUNT(*) calculate all rows even all columns data are NULL. However, COUNT(COLUMN_NAME) ignores NULL data.

e.g. distinct aggregate function
SQL>
SELECT
    distinct(commission_pct)
FROM
    employees;

COMMISSION_PCT
--------------
          NULL
           ,15
           ,35
            ,4
            ,3
            ,2
           ,25
            ,1

8 rows selected.

Info: DISTINCT(COLUMN_NAME) function uses not to see duplicated rows and also it shows NULL column if it exists.

e.g. both of count and distinct aggregate functions
SQL>
SELECT
    COUNT(DISTINCT(commıssıon_pct) )
FROM
    employees;

COUNT(DISTINCT(COMMISSION_PCT))
-------------------------------
                              7

Info: count(column_name) ignores NULL values, even if distinct function is used.