I was thinking about combining category numbers in a string, and putting
that in one field in the user table. Do you think that will be fast and
reliable?
I did this for www.MusicThoughts.com and it worked really well.
Each "thought" can have multiple categories.
There are 12 categories now, for example, but soon there might be 30.
I just gave each category a letter, and made a small "varchar()" field to store the string in.
So if thought id#231 is in categories a, h, p, and y, then its category field just says "ahpy"
If you allow each user a maximum of 7 categories, for example, you'd only need a "varchar(7)" field.
It's VERY easy to pass this in a URL and then separate it into an array on any page that needs it.