Main reason why I'd choose NULL is a bit simpler queries - you don't need to care about self-relationship when you are querying ,for instance, number of referred users by the given. You can also save some space in case you create an index on client_id (it usually makes sense) because some RDMS, for example Oracle, don't include NULLs into index, others, like SQL server, allow filtered indexes (WHERE client_id IS NOT NULL). 

How to create properly tree-hierarchy
I've setup a router in my basement, and an access point in my attic floor. They are connected with a patch cable. Both have the same SSID and WPA2 security. Roaming does work. But, what I want to know is, if I need to set them on different channels, or do they need the exact same channel to share. I've read many community forums and manuals, but yet it is not clear what to do because they have different opinions.


Router: Fritz!Box 7270
Accesspoint: Netgear WN604

