Why will mySQL not return a row with a null value when selecting != to a string

Question

I have a table with a nullable varchar column. When selecting rows and specifying that I want rows with a value not equal to a given string, it does NOT return the rows where the value is null.

For example:

## if `value` is null, that row is ignored
SELECT * FROM test_table WHERE value != 'some string'

I'd like to understand why that's happening.

Example: http://sqlfiddle.com/#!2/83f0d/1

Answer 1

In ANSI SQL, NULL is neither equal to nor unequal to any value, including itself.

NULL = 'foo'
NULL != 'foo'
NULL = NULL
NULL != NULL

all evaluate to NULL. To test for nullness you must use is null or is not null in your query.