Hm - I'm actually kinda surprised that works. I would have thought it would fail because the WHERE condition would evaluate AS

@TableB_ID IS NULL (TRUE) OR TableB_ID (Value OR NULL) = @TableB_ID (NULL)

I would have thought that the second part of that would evaluate to undefined, since you are comparing a NULL value with an equals operator. Then, since that one is undefined, the whole condition would evaluate to undefined, since you're ORing TRUE with undefined.

I guess that the second condition though, will instead evaluate as FALSE instead of undefined, and thus allow it to work?