Hi everyone,
I'm trying to calculate an output column which is the difference of two
other columns in the query output; the first column is an aggregate of
items in stock, while the second column is an aggregate of items which
have been used. The third column should should be the difference of the
two values so I can then output all three columns in a table.
Unfortunately I can't get this to work at the moment :(. I've simplified
the query down to the following:
dev=# select 1 as a, 2 as b, (b - a) as c;
ERROR: column "b" does not exist
dev=#
Do I need to create some form of alias so the calculation can see the
other columns? I am using PostgreSQL 7.4.2 on Linux.
Many thanks,
Mark.
---
Mark Cave-Ayland
Webbased Ltd.
Tamar Science Park
Derriford
Plymouth
PL6 8BX
England
Tel: +44 (0)1752 764445
Fax: +44 (0)1752 764446
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On Fri, Jul 16, 2004 at 15:31:33 +0100,
Mark Cave-Ayland <m.***********@ webbased.co.uk> wrote: Hi everyone,
I'm trying to calculate an output column which is the difference of two other columns in the query output; the first column is an aggregate of items in stock, while the second column is an aggregate of items which have been used. The third column should should be the difference of the two values so I can then output all three columns in a table.
Unfortunately I can't get this to work at the moment :(. I've simplified the query down to the following:
dev=# select 1 as a, 2 as b, (b - a) as c; ERROR: column "b" does not exist dev=#
Do I need to create some form of alias so the calculation can see the other columns? I am using PostgreSQL 7.4.2 on Linux.
You can't use column aliases in other columns; you need to repeat the
column expressions.
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On 16/07/2004 15:31 Mark Cave-Ayland wrote: Hi everyone,
I'm trying to calculate an output column which is the difference of two other columns in the query output; the first column is an aggregate of items in stock, while the second column is an aggregate of items which have been used. The third column should should be the difference of the two values so I can then output all three columns in a table.
Unfortunately I can't get this to work at the moment :(. I've simplified the query down to the following:
dev=# select 1 as a, 2 as b, (b - a) as c; ERROR: column "b" does not exist dev=#
Do I need to create some form of alias so the calculation can see the other columns? I am using PostgreSQL 7.4.2 on Linux.
I think you can use a sub-select (this works for me on 7.3.4):
select a, b, (b - a) as c from (select .... as a, .... as b from mytable)
as sub;
HTH
--
Paul Thomas
+------------------------------+---------------------------------------------+
| Thomas Micro Systems Limited | Software Solutions for
Business |
| Computer Consultants | http://www.thomas-micro-systems-ltd.co.uk |
+------------------------------+---------------------------------------------+
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Try
select a, b, (b - a) as diff from (
select 1 as a, 2 as b
) as tmp;
John Sidney-Woollett
Bruno Wolff III wrote: On Fri, Jul 16, 2004 at 15:31:33 +0100, Mark Cave-Ayland <m.***********@ webbased.co.uk> wrote:
Hi everyone,
I'm trying to calculate an output column which is the difference of two other columns in the query output; the first column is an aggregate of items in stock, while the second column is an aggregate of items which have been used. The third column should should be the difference of the two values so I can then output all three columns in a table.
Unfortunate ly I can't get this to work at the moment :(. I've simplified the query down to the following:
dev=# select 1 as a, 2 as b, (b - a) as c; ERROR: column "b" does not exist dev=#
Do I need to create some form of alias so the calculation can see the other columns? I am using PostgreSQL 7.4.2 on Linux.
You can't use column aliases in other columns; you need to repeat the column expressions.
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TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match
Mark Cave-Ayland wrote: Hi everyone,
I'm trying to calculate an output column which is the difference of two other columns in the query output; the first column is an aggregate of items in stock, while the second column is an aggregate of items which have been used. The third column should should be the difference of the two values so I can then output all three columns in a table.
Unfortunately I can't get this to work at the moment :(. I've simplified the query down to the following:
dev=# select 1 as a, 2 as b, (b - a) as c; ERROR: column "b" does not exist dev=#
Do I need to create some form of alias so the calculation can see the other columns? I am using PostgreSQL 7.4.2 on Linux.
You can can try:
select a, b, a-b from
( select sum( x) as a, sum( y) as b from whatever group by z);
You can also do:
select sum( x), sum( y), sum(x-y) from whatever group by z;
HTH Many thanks,
Mark.
---
Mark Cave-Ayland Webbased Ltd. Tamar Science Park Derriford Plymouth PL6 8BX England
Tel: +44 (0)1752 764445 Fax: +44 (0)1752 764446
This email and any attachments are confidential to the intended recipient and may also be privileged. If you are not the intended recipient please delete it from your system and notify the sender. You should not copy it or use it for any purpose nor disclose or distribute its contents to any other person. ---------------------------(end of broadcast)--------------------------- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
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On Fri, 16 Jul 2004 12:04:54 -0400, Tom Lane <tg*@sss.pgh.pa .us> wrote: select sum(x), sum(y), sum(x)-sum(y) from ...
At least since 7.4, the system will notice the duplicate aggregates and run only two summations to compute the above, followed by a single subtraction at the end. The apparently more intelligent way suggested by Jean will have to run three summations, and thus end up being a net loss.
Also note that Jean-Luc's
select sum( x), sum( y), sum(x-y) from whatever group by z;
gives a different result in the presence of NULLs.
Servus
Manfred
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