Small documentation issue, visible in web documentation for SQL Language Expressions as of 23 November 2021: In both (a) the syntax diagram for expressions, and (b) the list of Operators, it seems that the use of 'IS NULL' (two separate words) is not permitted. But I know it is in fact permitted. If it were just in the syntax diagram I would think that I had failed to understand it somehow, but the list of Operators makes very clear the contrast between the two forms allowed for 'not null' and the one form allowed for 'null'.