MySQL COUNT() and nulls

Question

Am I correct in saying:

COUNT(expr)
WHERE expr IS NOT *  

Will count only non nulls?

Will COUN

Answer 1

If you want to count NULLs as well, try

SELECT COUNT(IFNULL(col, 1)) FROM table;

Answer 2

just checked:

select count(*)

returns 1 with one record filled with NULLs

select count(field)

returns 0.

I don't see the point in the record with NULL values. Such record must not exist.

Answer 3

Correct. COUNT(*) is all rows in the table, COUNT(Expression) is where the expression is non-null only.

If all columns are NULL (which indicates you don't have a primary key, so this shouldn't happen in a normalized database) COUNT(*) still returns all of the rows inserted. Just don't do that.

You can think of the * symbol as meaning "in the table" and not "in any column".

This is covered in the MySQL Reference Manual.

Answer 4

Using MySQL I found this simple way:

SELECT count(ifnull(col,1)) FROM table WHERE col IS NULL;

This way will not work:

SELECT count(col) FROM table WHERE col IS NULL;

Answer 5

count(*) is not for non-null columns, it's just the way to ask to count all rows. Roughly equivalent to count(1).