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.
