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Re: Referncing a calculated column in a select?

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On 2019-09-12 15:35:56 -0500, Ron wrote:
> On 9/12/19 2:23 PM, stan wrote:
> > I am creating some views, that have columns with fairly complex calculations
> > in them. The I want to do further calculations using the result of this
> > calculation. Right now, I am just duplicating the first calculation in the
> > select fro the 2nd calculated column. There must be a batter way to do
> > this, right?
> 
> I must be misunderstanding you, because you can just do more of what you're
> doing now.
> 
> Here's an example of doing some calculations in the creation of the view,
> and then more calculations when selecting from the view:

I think he wants to refer to other columns in the view. 

Something like

    create or replace view v as
        select a,
               b,
               abs(b - a)  as abserror,
               case when abs(a) > abs(b) then abs(a) else abs(b) end as mag,
               abserror / mag as relerror
        from t;

Except that this doesn't work.

What you can do is nest views:

    create or replace view v as
        select a,
               b,
               abs(b - a)  as abserror,
               case when abs(a) > abs(b) then abs(a) else abs(b) end as mag
        from t;

    create or replace view v2 as
        select a,
               b,
               abserror,
               mag,
               abserror / mag as relerror
        from v;

wds=> select * from v2; 
╔══════════════╤═════════╤══════════════╤══════════════╤═══════════════════╗
║      a       │    b    │   abserror   │     mag      │     relerror      ║
╟──────────────┼─────────┼──────────────┼──────────────┼───────────────────╢
║            2 │       3 │            1 │            3 │ 0.333333333333333 ║
║           -2 │       3 │            5 │            3 │  1.66666666666667 ║
║ 3.1415926536 │ 2.71828 │ 0.4233126536 │ 3.1415926536 │ 0.134744602587137 ║
╚══════════════╧═════════╧══════════════╧══════════════╧═══════════════════╝
(3 rows)

No idea whether this is more or less efficient than writing the whole
formula for each column.

        hp

-- 
   _  | Peter J. Holzer    | we build much bigger, better disasters now
|_|_) |                    | because we have much more sophisticated
| |   | hjp@xxxxxx         | management tools.
__/   | http://www.hjp.at/ | -- Ross Anderson <https://www.edge.org/>

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