MySQL NULLs I say almost any database because MySQL has an odd quirk. NULL is treated as a unique value — which is why you cannot use comparisons such as value = NULL and need to use value IS NULL. Unfortunately, this also affects unique indexes and no logic has been implemented to fix it.
We can execute our original INSERT multiple times and a new record will be created each time because the extension field defaults to NULL and is considered to be unique:
(1, 234, 567890);