Why COUNT() doesn’t count NULL values in MySQL?

Question

Hey everyone 👋
I’m learning SQL and got a small doubt.
COUNT() counts rows, but it skips NULL values.
Why does COUNT() ignore NULL?
And also, how does COUNT(IF(...)) work when there are NULLs?

Here’s a small example 👇

CREATE TABLE students (
id INT,
marks INT
);

INSERT INTO students VALUES
(1, 90),
(2, NULL),
(3, 80);

SELECT COUNT(marks) AS count_marks,
COUNT(*) AS count_rows
FROM students;

Output:
count_marks → 2
count_rows → 3

Can someone explain why COUNT(marks) = 2 instead of 3?

Answer 1

COUNT() ignores NULL because NULL means “no value.” COUNT(marks) counts only rows where marks is not NULL, while COUNT(*) counts all rows.
So in your example, marks has 2 real values and 1 NULL → result is 2 vs 3.

Solution:
If you want to treat NULL as 0 or count them too, use: COUNT(IFNULL(marks, 0))
or simply use: COUNT(*) -- counts all rows including NULLs
Good luck!