What generic techniques can be applied to optimize SQL queries?

Question

What techniques can be applied effectively to improve the performance of SQL queries? Are there any general rules that apply?

Answer 1

Make sure that you have the right indexes on the table. if you frequently use a column as a way to order or limit your dataset an index can make a big difference. I saw in a recent article that select distinct can really slow down a query, especially if you have no index.