Question

X is column with a mixture of numerical and null values.

I don’t understand why treating null values as 0 changes the result of the AVG value.

Shouldn’t it be like this,

(2 + null + 2) / 3 = 2 -- with null value

(2 + 0 + 2) / 3 = 2     -- no null value

Answer 1

If you have null value, it is not being taken into account, hence number of elements (your n) is not increased, whereas 0 is treated as a valid value.

For example if you have: 1,1,6,7,null,3,2 it will be total 20 divided by 6 (i.e. 3.33). However if you replace null with 0 it becomes 20 /7 (i.e. 2.86).

SQLFiddle demonstrating this behavior on MySQL server.

REFERENCE: Have a look in the documentation right below the table:

This section describes group (aggregate) functions that operate on sets of values. Unless otherwise stated, group functions ignore NULL values.