Обсуждение: How can I re-use an expression in a SELECT?
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
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
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
) subquery
Surely there are more elegant ways of doing this in later versions.
I'm still using 8.3 ...
Best,
Oliveiros
----- Original Message -----From: Rob RichardsonSent: Friday, July 06, 2012 2:04 PMSubject: [NOVICE] How can I re-use an expression in a SELECT?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
I understand. Thanks very much!
RobR
From: Oliveiros d'Azevedo Cristina [mailto:oliveiros.cristina@marktest.pt]
Sent: Friday, July 06, 2012 9:30 AM
To: Rob Richardson; pgsql-novice@postgresql.org
Subject: Re: [NOVICE] How can I re-use an expression in a SELECT?
I don't know any way of doing it without using a sub-query...
SELECT 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
) subquery
Surely there are more elegant ways of doing this in later versions.
I'm still using 8.3 ...
Best,
Oliveiros
----- Original Message -----
From: Rob Richardson
Sent: Friday, July 06, 2012 2:04 PM
Subject: [NOVICE] How can I re-use an expression in a SELECT?
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
----- Original Message -----From: Rob RichardsonSent: Friday, July 06, 2012 2:45 PMSubject: Re: [NOVICE] How can I re-use an expression in a SELECT?I understand. Thanks very much!
RobR
From: Oliveiros d'Azevedo Cristina [mailto:oliveiros.cristina@marktest.pt]
Sent: Friday, July 06, 2012 9:30 AM
To: Rob Richardson; pgsql-novice@postgresql.org
Subject: Re: [NOVICE] How can I re-use an expression in a SELECT?
I don't know any way of doing it without using a sub-query...
SELECT 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
) subquery
Surely there are more elegant ways of doing this in later versions.
I'm still using 8.3 ...
Best,
Oliveiros
----- Original Message -----
From: Rob Richardson
Sent: Friday, July 06, 2012 2:04 PM
Subject: [NOVICE] How can I re-use an expression in a SELECT?
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