NULL Semantics

Most comparisons to null return null. Exceptions are generally in functions or comparisons specifically to handle null, such as IS NULL.

When the outcome of a comparison is null, this will be coerced to false when used in a filter (WHERE or HAVING) but return as null in a SELECT statement.

To demonstrate, first a null comparison in a SELECT statement:

SELECT name = null
 FROM $planets;

This returns null for all values.

 name=null
-----------
 null
 null
 null
 null
 null
 null
 null
 null
 null

Using a null comparison in a WHERE statement will always result in false.

SELECT name
  FROM $planets
 WHERE name = null;

Returns an empty set, this is true even for inequality tests:

SELECT name
  FROM $planets
 WHERE name != null;

Also, returns an empty set.

Be Aware

null comparison returning null holds true even for null = null. Do not test for null using an equals condition, use IS NULL.