Re: Parsing a Calculation from a field - Mailing list pgsql-sql

Thanks for your help Richard,

I tried a number of Pl/SQL approaches, but couldn't get through the type-checking.  I finally knocked out a little
PL/TCLscript that seems to get the job done:
 

create function eval_sums(varchar(8000)) returns integer as ' if {[argisnull 1]} { return_null } return [expr $1]
'
language pltcl;


Hopefully this might help out anyone else faced with a similar problem.

Kieran


-----Original Message-----
From: Richard Huxton [mailto:dev@archonet.com] 
Sent: 11 January 2005 11:35
To: Kieran Ashley
Cc: pgsql-sql@postgresql.org
Subject: Re: [SQL] Parsing a Calculation from a field

Kieran Ashley wrote:
> 
> Some of our data comes in in a format which provides us with extra
> information, but which we currently don't need to use; specifically
> we sometimes receive information in the form of calculations, for
> example a column which needs to be transformed to an integer is
> initially of type varchar, and contains values like:
> 
> 
> 
> 6
> 10
> 2
> 4 + 8
> 2 + 4 + 8
> NULL
> 4
[snip]
> I could obviously add an extra step to the procedure and write a
> script using something like PHP to scan the table and carry out any
> necessary calculations in advance, but I'm curious to know if there
> is a way within Postgres to do it either as part of the select query
> itself, or as a PL/SQL procedure.  I've had a quick look at the
> string handling functions but I can't see any way to parse integers
> out of a varchar column - which would seem to prohibit using
> substring to manually break up the sums.

You should look into eval() - available in most scripting languages in 
some form or other. It treats its parameter as code/an expression and 
evaluates it. WARNING - can be vulnerable to abuse, make sure you trust 
or clean your input data first.

You could probably do it in pl/perl or pl/perlu, pl/tcl. Perhaps pl/php 
too. The only way I can think to do it in pl/pgsql would be to have a 
line like:
  eval_qry := ''SELECT ('' || $1 || '')::integer AS result''

Then use FOR..IN..EXECUTE to get the results.

HTH
--  Richard Huxton  Archonet Ltd

#############################################################

The information contained in this email and any subsequent
correspondence is private and is intended solely for the 
intended recipient(s). For those other than the intended
recipient(s) any disclosure, copying, distribution, or any 
action taken or omitted to be taken in reliance on such 
information is prohibited and may be unlawful.

#############################################################