How to store bidirectional relationships in a RDBMS like MySQL?

Question

Suppose I want to store relationships among the users of my application, similar to Facebook, per se.

That means if A is a friend(or some relation) of B

Answer 1

Storage is relatively cheap these days, so I would not worry about it because of that.

What would concern me is that you must now clean up as you are storing the information twice. So if you "unfriend" someone, you have to remove 2 records, not just one.

The other considerations are searches and indexing. There could be advantages of hashing the combination of 2 users ids to check for existance, provided you follow a consistant convention (like always append the higher id to the lower before hashing).

So now you have other possibilities. Are you interested on querying the relationship between the 2 users? Or is it more important to look at the attributes of one user?

These are concerns about what the system will do. Take a look at subjects like DDD (Domain Driven Design) and CQRS (Command Query Responsibility Segregation) to see how to divide up your app so each area is implemented in the simplest way possible. This will give you avenues to fine tune and optimize later without running into complexity issues.