Just for clarification: You edit the forum's info and move it to a sub-forum. In the database there is now a number for F_PARENT but the forum itself is showing as a regular forum and not a sub-fourm.
Unless I'm missing something, if F_PARENT has a value it shouldn't be showing.... I've cross-referenced your code with mine. Not to belabor the point, but in the database you're sure that there is a value for F_PARENT? I only ask because the SQL statement is set up so that if F_PARENT is not null it won't pull that forum.
Outside of double-checking that I'm running short of ideas as to what the problem could be.... I'm fairly certain that the "WHERE F.F_PARENT IS NULL" works the same on MySQL as it does on Access and SQL Server....
And not to be insulting, but are you checking the right database? There's been more than a few times that I was unsuccessfully hunting a problem in the live environment only to realize that I was looking at the database for the test environment... and at one point I had accidentally had them both set up on the same db... (was that ever fun )<
I'll have to really meditate on this one.... I can't see any reason why that wouldn't work....
Try making the changes I mentioned in the here. It will correct the statistics at the bottom and may give some new insight into what the problem could be. Just keep in mind that your line numbers are a little different.
Of course, it could just be me. I'm working on getting the "jump to last post" to link to one of the sub-forums if a subforum has a newer post than the parent forum... and yet I'm getting the oddest error... for the life of me I can't seem to find it either. What a Wed. this has been.<