Re: Referncing a calculated column in a select?

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

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