I think that PVD always stores a numeric zero for unrated movies, even if that zero isn't displayed in Edit Mode, which is why "= 0" and "IS NULL" are equivalent for "Rating"
For me, this is weak spot and I'll be free to try to clarify it further.
If you try to assign value "0" to "Rating" in edit mode you won't succeed, it'll silently become blank. Why? Isn't rating a number field, and isn't zero a number? The only logical reason for me why is that so is that it was meant for value "0" to mean "Unrated" and that "Unrated" is treated like NULL, and that's how "0" became "NULL".
How I see things, Value was equaled with Condition.
Orescb already noticed this
here 13months ago and I found this after we conclude it here almost exactly the same as he was back then.
Nostra asked his suggestion, he responded, but no feedback, neither changes in PVD.
The fact that after 13 months a "new generation" of devoted users again recognized this as an possible issue should put this on "to revise list".
My suggestion for "Rating"
0. Default state is blank (NULL)
1. "0" is allowed (Value)
2. "Blank" is allowed (NULL)
3. Rating in View mode means that the one wants REALLY to rate the movie. So, any action in view mode can produce only values (0-10).
4. If one wants to "unrate" the movie, he could do it only in Edit mode by clearing rating - "blank"
Other numeric fields:
5. Default box office is blank (NULL) - We still don't know it
6. "0" is allowed (Value) - The producers committed suicide
7. "Blank" is allowed (NULL) - We don't want to know, or we aren't wanted to know how reach they became. (it isn't allowed at the moment!)
This way, all numeric fields would act identically. They would all have the same "bug" (ref. Aims testings). And I'm afraid this would be only the start point for the questions of all questions:
How's 0=NULL?*
*Or "Where's the pie?", or "Where's the money?"