Strange behavior when checking for Null values in Access SQL [duplicate]

Question

This question already has answers here:

Inconsistent results when checking for Null values (Jet DAO vs. ACE DAO) (2 answers)

Closed 8 years ago.

I am having an issue with filtering out phone numbers which have no value in them ie:

(___) ___ - ____

I tried this in my WHERE clause to get ALL the values that had a full phone number:

WHERE WRT_Phone <> null;

This allowed me to get all the phone numbers that had a full value in them. But, when I attempt

WHERE WRT_Phone = null;

Nothing gets populated in the resulting table. Are there any functions that I can use to turn the area code into a number or a function to check if phone number data type is null?

Also, I am using Access 2010 for executing the SQL statements.

Answer 1

The syntax to check for nulls is:

WHERE WRT_Phone IS null;

Answer 2

To check for nulls you need to use IS NULL:

WHERE WRT_Phone IS NULL

In fact, to check for not null, you should technically use IS NOT NULL:

WHERE WRT_Phone IS NOT NULL

Something frankly odd (unique to Access?) is happening here because you should not be getting any results with WHERE WRT_Phone <> null either.

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Generally speaking, when NULL is present on either side of a boolean operator or as part of an expression, the whole thing evaluates to NULL (which is considered FALSE for the purposes of a WHERE clause):

• In a non-null record:
  • WRT_Phone = NULL expands to 'foobar' = NULL which equals NULL -> no record returned
  • WRT_Phone <> NULL expands to 'foobar' <> NULL which equals NULL -> no record returned
• Likewise, in a null record:
  • WRT_Phone = NULL expands to NULL = NULL which equals NULL -> no record returned
  • WRT_Phone <> NULL expands to NULL <> NULL which equals NULL -> no record returned