I discovered the SELECT NULL syntax by observing the internal SQL statements issued by Oracle when it is checking FK constraints. So if it was good syntax for the engine, then it must be good for us programmers.

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Side note: Both DB2 and Oracle, both architected from the same original System R specification, use SQL to run the engine. This was a good thing, in that it forced optimization as well as enhancements to the SQL language.

One of the first things that DB2 encountered was heavy lock contention and deadlocking on its own internal dictionary tables as it was taking page-level locks. The next release (circa 1984) had row-level locking.