... I know this is totally incorrect, because this way will always leave a trailing comma.

So just do chop $SQL; or $SQL =~ s/,$//; to get rid of the comma, before you add the next part of the statement ($SQL .= " FROM ...").

But I think the sample code you just gave is a bit off track. Let's go back to your CGI param string (from the OP): 

../cgi-bin/script.pl?column3=1&column2=2&column1=3

[download]
If you get your params like this: 
my $q = new CGI;
my $href = $q->Vars;

[download]
Then you can form the string of column names like this: 
my $col_string = join( ',', sort {$$href{$a} <=> $$href{$b}} keys %$hr
+ef );

[download]
I don't think you'll ever need to worry about any extra commas with that approach, but if you want to be totally certain: 
$col_string =~ s/^,+//;      # remove leading commas
$col_string =~ s/,+$//;      # remove trailing commas
$col_string =~ s/,{2,}/,/g;  # reduce adjacent commas to ","

[download]
Or, using something more like your approach, just append column names in order, and get rid of the last comma: 
my $col_string = '';
for my $var ( sort {$$href{$a} <=> $$href{$b}} keys %$href ) {
    $col_string .= "$var,";
}
chop $col_string;  # remove final comma

[download]
Either way, now you just finish the SQL statement: 
my $sql = "SELECT $col_string FROM my_table";

[download]
Also, in all cases, if/when you decide (or realize) that you do need to worry about security issues, make sure to test the "$href" keys (assumed table column names) against a known (untainted) list that is based directly on the how the table was defined. Delete any hash keys from the CGI parameter string that do not exactly match a known table column.

This sort of dynamic query construction is an area where things can go horribly wrong if you are not meticulously careful about checking your tainted input. If you don't have "-T" on the shebang line of your script (to turn on taint-checking), add it now.

This is what I am doing with the XML::Generator, but the stuff it returns is hard coded, and not dynamic variables...

That's why I say you should use XML::Simple (or something else besides XML::Generator) -- you need to write XML data based on a data structure, where the highest (outermost) level of the structure is a hash.

In reply to Re^3: Building SQL Query on the fly by graff
in thread Building SQL Query on the fly by hallikpapa

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.