Re: How can I re-use an expression in a SELECT?

Rob Richardson, 06.07.2012 15:04:
> Greetings!
>
> It seems to me, from my uneducated perspective, that if I have a
> calculated result with a name in a SELECT query, then I ought to be
> able to re-use the name later in the statement:
>
> select charge,
> case when c.rev_heat_time = 0 then c.pred_heat_time
> else c.rev_heat_time
> end as heat_time,
> case when c.rev_cool_time = 0 then c.pred_cool_time
> else c.rev_cool_time
> end as cool_time,
> fire_date + interval '1 minute' * heat_time as end_of_fire
> from charge c
>
> Of course, that fails because heat_time cannot be used to calculate
> end_of_fire.  I would have to repeat the case statement that defines
> it instead.  How can I avoid repeating complicated pieces of SELECT
> queries like this?
>
> Thanks very much!
>
> RobR
>

You need a derived table (aka sub-select):

select charge,
        heat_time,
        cool_time,
        fire_date +interval '1 minute' * heat_time as end_of_fire
from (
   select charge,
          CASE
            WHEN c.rev_heat_time = 0 THEN c.pred_heat_time
            ELSE c.rev_heat_time
          END AS heat_time,
          CASE
            WHEN c.rev_cool_time = 0 THEN c.pred_cool_time
            ELSE c.rev_cool_time
          END AS cool_time
   from charge c
) t