Question

This might be the situation in other databases as well but when you make the following query

SELECT * FROM MyTbl WHERE MyColumn != 'Foo'

then any record where MyColumn is, say, ‘Bar’ is fetched but not where MyColumn is NULL. I assume this is expected behavior and that there is a reason behind it and I’d like to know why.

Is NULL considered to be equal to ‘Foo’ or is it just not expected to be part of the condition because the condition (NULL != ‘Foo’) seems to be true.

Answer 1

In DB logic, NULL means that there is simply no defined data in this field. It’s considered neither equal or different to anything. You have to filter on it explicitly if you want to fetch the relevant lines :

SELECT * FROM MyTbl WHERE MyColumn != 'Foo' OR MyColumn IS NULL

See Wikipedia.