Thread: referencing to "computed columns" in where clause

Is there a way to do things like

select (a-b) as pippo from ... where pippo=7;
or
select
  case
    when (a>3) then a*b
    when (a<3) then a+b
  end as pippo
where pippo<12;

other than defining a function such that I can write:

select func(a,b,c) as pippo from ... where func(a,b,c)=7;
?

Is there anything else I can do to avoid duplication of code?


--
Ivan Sergio Borgonovo
http://www.webthatworks.it

On Tue, Jan 29, 2008 at 06:04:48PM +0100, Ivan Sergio Borgonovo wrote:
> select
>   case
>     when (a>3) then a*b
>     when (a<3) then a+b
>   end as pippo
> where pippo<12;

I've tended to do:

  SELECT *
  FROM (
    SELECT "complicated expression" AS pippo) x
  WHERE pippo  < 12;


  Sam

On Tue, 29 Jan 2008 17:17:39 +0000
Sam Mason <sam@samason.me.uk> wrote:

> On Tue, Jan 29, 2008 at 06:04:48PM +0100, Ivan Sergio Borgonovo
> wrote:
> > select
> >   case
> >     when (a>3) then a*b
> >     when (a<3) then a+b
> >   end as pippo
> > where pippo<12;
>
> I've tended to do:
>
>   SELECT *
>   FROM (
>     SELECT "complicated expression" AS pippo) x
>   WHERE pippo  < 12;

It risk to be much harder to rewrite for dumber DB than repeating the
"complicated expression".

Does it come with some extra cost/improvement in term of performance
compared to:
- repeating the code of "complicated expression"
- put it in a function with the proper "attributes" (I'd say
IMMUTABLE in the above case)

I've some argument for all cases but it should depend on the
implementation.

thanks

--
Ivan Sergio Borgonovo
http://www.webthatworks.it

--- On Tue, 1/29/08, Ivan Sergio Borgonovo <mail@webthatworks.it> wrote:

>> other than defining a function such that I can write:

   SELECT ( a-b ) as pippo
     FROM Yourtable
    WHERE ( a-b ) = 7
UNION ALL
   SELECT ( a*b ) as pippo
     FROM Yourtable
    WHERE ( a*b ) < 12
      AND a > 3
UNION ALL
   SELECT ( a+b ) as pippo
     FROM Yourtable
    WHERE ( a+b ) < 12
      AND a < 3;

On Tue, Jan 29, 2008 at 06:49:50PM +0100, Ivan Sergio Borgonovo wrote:
> On Tue, 29 Jan 2008 17:17:39 +0000 Sam Mason <sam@samason.me.uk> wrote:
> > I've tended to do:
> >
> >   SELECT *
> >   FROM (
> >     SELECT "complicated expression" AS pippo) x
> >   WHERE pippo  < 12;
>
> It risk to be much harder to rewrite for dumber DB than repeating the
> "complicated expression".

If you're worrying about how the various implementations handle the
cases then I'd suggest testing the code in each one and see how they
handle it.

> Does it come with some extra cost/improvement in term of performance
> compared to:
> - repeating the code of "complicated expression"

Most reasonable databases will expand subselects where possible.
Optimisation is always a tradeoff between different factors--one of
these being maintainability.

> - put it in a function with the proper "attributes" (I'd say
> IMMUTABLE in the above case)

That's almost never going to be a win; rewriting sub-selects is almost
always going to be easier than rewriting stored procedures.

> I've some argument for all cases but it should depend on the
> implementation.

My recommendation is generally to use the easiest solution that works
now and only worry about things when they actually fail.


  Sam