Expressions in queries

[mjac]

I asked this last night on quakenet and didn't get an answer (as par usual).

How can I do something similar to: SELECT (complicated expression) as `exp` WHERE `field`<`exp` and `anotherfield`>`exp`.

I want to use exp more than once, but I don't want it to be calculated many times. writing the full expression for each use of exp is labourous and slow. Any ideas?

Thanks