NULL

A recent question from OTN:

What’s the difference between “{expression} is null” and “{expression}= null” ?

Technically we can say the following:

• {expression} is null will evaluate to one of TRUE or FALSE
• {expression} = null will evaluate to NULL

As a follow on from this: a predicate applied to a dataset will return rows only when it evaluates to TRUE; a constraint will allow rows to be inserted into a table if if doesn’t evaluate to FALSE.

Three-valued logic catches everybody out occasionally.