Not equal <> != operator on NULL

Question

Could someone please explain the following behavior in SQL?

SELECT * FROM MyTable WHERE MyColumn != NULL (0 Results)
SELECT * FROM MyTable WHERE MyColumn <         


        

Answer 1

Old question, but the following might offer some more detail.

null represents no value or an unknown value. It doesn’t specify why there is no value, which can lead to some ambiguity.

Suppose you run a query like this:

SELECT *
FROM orders
WHERE delivered=ordered;

that is, you are looking for rows where the ordered and delivered dates are the same.

What is to be expected when one or both columns are null?

Because at least one of the dates is unknown, you cannot expect to say that the 2 dates are the same. This is also the case when both dates are unknown: how can they be the same if we don’t even know what they are?

For this reason, any expression treating null as a value must fail. In this case, it will not match. This is also the case if you try the following:

SELECT *
FROM orders
WHERE delivered<>ordered;

Again, how can we say that two values are not the same if we don’t know what they are.

SQL has a specific test for missing values:

IS NULL

Specifically it is not comparing values, but rather it seeks out missing values.

Finally, as regards the != operator, as far as I am aware, it is not actually in any of the standards, but it is very widely supported. It was added to make programmers from some languages feel more at home. Frankly, if a programmer has difficulty remembering what language they’re using, they’re off to a bad start.