[zer00eyz]

Go read: Database Design for Mere Mortals.

ERD's are your friend. Learn how to generate one, and how to read it.

The relations (not relational, not algebra) are IN the design they are IN the ERD (as a tool to visualize). Even if your not visual thinker the ERD might help you find a path between two distant tables.

Needing a subquery is rare. It happens but a lot of subqueries would be better off as joins. The moment you grasp the design of something you're less likely to want to sub query.

Explain is your friend. Reading an explain plan is going to give you some good insight into what is going on UNDER the hood. Not only will it help you tune slow queries but it is more insight into how large queries decompose.

Lastly, there is nothing worse than having to query a badly designed DB. If you do a shit job on the first part everything else is going to be painful.