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[vox-tech] Re: vox-tech Digest, Vol 32, Issue 9
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[vox-tech] Re: vox-tech Digest, Vol 32, Issue 9



The "EXECUTE" command is typically used on complete sql expressions.
(the information is parsed, and passed to the database.)

my_command := 'select count(*) from my_table'
EXECUTE my_command;

So if you want to dynamically select a column.

my_column := 'DUMMY'
my_command := 'select '||my_column||' from my_table;'
EXECUTE my_command;

Be careful using dynamic sql on web pages.  The unchecked
concatenation of strings can lead to "SQL Injection" vulnerabilities.

Kevin

On 1/10/07, vox-tech-request@lists.lugod.org
<vox-tech-request@lists.lugod.org> wrote:
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Today's Topics:

   1. pl/pgsql question: dynamic access to record.column
      (Dylan Beaudette)


----------------------------------------------------------------------

Message: 1
Date: Wed, 10 Jan 2007 11:28:47 -0800
From: Dylan Beaudette <dylan.beaudette@gmail.com>
Subject: [vox-tech] pl/pgsql question: dynamic access to record.column
To: vox-tech@lists.lugod.org
Message-ID: <200701101128.47428.dylan.beaudette@gmail.com>
Content-Type: text/plain;  charset="us-ascii"

Hi,

I am learning how to use PL/PGSQL and have come upon something that I just
don't seem to be able to solve on my own.

I have a function, which takes a column name as an argument. A FOR loop
iterates over rows, using a RECORD datatype to store each iteration of the
loop.

If I hard-code in a column name into some calculation, things work fine.
However, I cannot seem to figure out how to _dynamically_ access a column
from a RECORD datatype:

-- this works fine : 'field_pct_clay' is a hard-coded column name
DEFINE
hz_record RECORD;
wt_column FLOAT;
thick FLOAT;
...
wt_column := wt_column + (hz_record.field_pct_clay * thick) ;

-- this does not work: 'the_column' is an argument to the function being
defined:

wt_column := wt_column + (hz_record.the_column * thick) ;
                                                            ^^^^^^^^^^^^^

obviously, the interpreter tells me that there is no column named 'the_column'
in the RECORD iterator...

Searching on google, it seems that a dynamic expression like this needs to be
evaluated with the EXECUTE keyword.... However something like this does not
work:

wt_column := EXECUTE 'wt_column + (hz_record.' || quote_ident(the_column) || '
* thick)' ;

any tips would be a great help!

thanks!

--
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341


------------------------------

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End of vox-tech Digest, Vol 32, Issue 9
***************************************


--
-- Kevin Hrim

/*
Teach this triple truth to all: A generous heart, kind speech, and a
life of service and compassion are the things which renew humanity. -
Buddha
*/
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