How to Write Optimal SQL Queries

Question

I\'ve searched around stackoverflow but everybody asks to optimize queries they\'ve already done.

I want to know, the basic stuff on what to do, what to avoid when creat

Answer 1

• Views are macros, not magic
• EXISTs and NOT EXISTs work best usually
• Functions on columns (see Joel C's answers)
• Beware implicit conversion (eg smallint column compared to int parameter)
• Understand covering indexes
• Denormalise after you see issues
• Understand aggregates: stop thinking of loops
• ...

Edit, Feb 2012:

Avoid these "Ten Common SQL Programming Mistakes"