I worked at a company where we used a decimal number to represent a tree for a threaded forum. It meant that we could use an ORDER BY clause and quickly get al

I worked at a company where we used a decimal number to represent a tree for a threaded forum. It meant that we could use an ORDER BY clause and quickly get all the descendants sorted, like this: 7.401 7.40101 7.40102 7.4010201 7.40103 7.4010301 The downside was that it wasn't nearly this straightforward -- numbers seemed to be assigned haphazardly. And the scheme wasn't documented well, so it was nearly impossible for new programmers to understand. I would not recommend this approach again. If you need high-performance searches for descendants, a separate table relating ancestors to descendants will be much more straightforward in the long-run.