I found the problem and immediately wrote a script to recover most of the tables from an old backup I made awhile back. The 400 topics that could not be recovered automatically had to be manually recovered. Basically, I went through all those 400 topics and assigned the correct value to the aforementioned column in them individually.
It wasn't an easy task because sometimes the topic title and topic description can be so vague that you just have no idea what section they belong to. Thus, I have to go in to each topic manually and skim through a few posts to get an idea what the topic is about. As a result, some topics might not get assigned to their original section. If you see a topic that doesn't belong to the correct section, please contact the staff and ask them to move it to the correct location. Thank you very much for your cooperations and your patience. And I apologize for the inconvenience you may have experienced on the forums during these times.
The moral of the story here is, always make backup copies of your data and don't mess with SQL when you are tired. You could make a very small mistake that end up in catastrophic results.
P.S. - If you have missed what happened, here is a screenshot for reference.
