return string from query (DBI, MySQL) got truncated when trying to generating statement dynamically for MySQL

[khandielas]

Hi, all, I could not find the solution for my problem.

I try to pivot several mySQL tables for reporting, the row number is not fixed so what I am doing now is that grouping it first as a subquery and then pivot it. It works fine with small number of rows but does not work with large number rows.

Here is an example, their are many cols, but only prod matters to me.

prod       xx  xx  ...
'prod 1'   
'prod 2'   
'prod 1'   
'prod 3'
'Prod 4'   
......

After grouping by prod, the table should be

prod    count
prod 1   10
prod 2   15
......

Here is my code:

use DBI;
my $dbh = connect(....);

my $pvt_col = 'prod';
my $count = 'count';
my $table = 'prod'

my $stmt = q{
SELECT
  CONCAT( 
    '(SUM(CASE WHEN }. $pvt_col. q{ IS NULL THEN Count ELSE 0 END)) AS
+ Total, ',
    GROUP_CONCAT(DISTINCT  
        CONCAT(
         '(SUM(CASE WHEN }. $pvt_col. q{ = ''', }. $pvt_col. q{, ''' T
+HEN ', }. $count. q{, ' ELSE 0 END)) AS `', }. $pvt_col. q{, '`'
        )
    )
  ) 
FROM         
   ( SELECT }. $pvt_col. q{, count(*) AS }. $count. 
  q{ FROM }. $table.
  q{ GROUP BY }. $pvt_col. q{ WITH ROLLUP) as temptbl;};

my $sth = $dbh->prepare($stmt);
$sth->execute();
my $tmp;
while ( ($tmp) = $sth->fetchrow_array() ) {
  print"$tmp\n";
  last;
}

The expected output should be:

(SUM(CASE WHEN prod IS NULL THEN Count ELSE 0 END)) AS Total,
(SUM(CASE WHEN prod = 'prod1' THEN 10 ELSE 0 END)) AS `prod1`,
(SUM(CASE WHEN prod = 'prod2' THEN 15 ELSE 0 END)) AS `prod2`,
(SUM(CASE WHEN prod = 'prod3' THEN 1 ELSE 0 END)) AS `prod3`,
.....

After this, I can query again to produce pivot results:

my $statement = 
    q{ SELECT }. $tmp. 
    q{ FROM( SELECT }. $pvt_col. q{, count(*) AS }. $count. 
          q{ FROM }. $table.
          q{ GROUP BY }. $pvt_col. q{ WITH ROLLUP
           ) as temptbl;};

$sth = $dbh->prepare($statement);
....

$statement is like:

SELECT (
(SUM(CASE WHEN prod IS NULL THEN Count ELSE 0 END)) AS Total, 
(SUM(CASE WHEN prod = 'prod1' THEN 10 ELSE 0 END)) AS `prod1`,
(SUM(CASE WHEN prod = 'prod2' THEN 15 ELSE 0 END)) AS `prod2`,
(SUM(CASE WHEN prod = .....
...
FROM( SELECT prod, count(*) AS Count FROM prod GROUP BY prod WITH ROLL
+UP) as temptbl;

It works beautifully for tables with small number of rows, but failed for tables with larger number of rows. $tmp is trucated when I have large rows. So $statement cannot be excuted successfully.

Monks, any help is highly appreciated!

Environment: Perl 5.14.2, ubuntu 12.04.2, MySQL 5.5.31

[khandielas]

Another question is whether this is the right way to do pivot for MySQL table? The output from above code is something like:

total prod 1  prod 2 .....
585    10      15

I also use the same technique for percentage column too.

[poj]

It looks easier with a hash

#!perl
use strict;
use DBI;
my $dbh = connect(..);

my %pivot=();
my $total;

my $sql = 'SELECT prod,count(*) FROM prod GROUP BY prod';
my $sth = $dbh->prepare($sql);
$sth->execute();
while (my @f = $sth->fetchrow_array){
  $pivot{$f[0] || 'null' } = $f[1];
  $total       += $f[1];
}     
my @cols = sort keys %pivot;
unshift @cols,'total';
$pivot{'total'} = $total;

print sprintf "| %5s ",$_ for @cols;
print "|\n";
print sprintf "| %5d ",$pivot{$_} for @cols;
print "|\n";

Note ; This assumes you don't have a prod called 'total'.
poj

[khandielas]

Thank you. Using perl code to process the result sets is a solution, but I really want to know what is the reason for the failure and what parameter in DBI or MySQL control the result string length.

I like to let Database engine to do all the work if possible, esp. for any type of 'set' calculation, so that I don't need to process row by row. Your code can be simplified a bit with 'ROLLUP' in the query':

my $stmt = "SELECT prod, count(*) from prod GROUP BY prod WITH ROLLUP;
+";

$sth = $dbh->prepare( $stmt );
$sth->execute();

my %pivot;
while ( my ( $k, $v ) = $sth->fetchrow_array() ) {
   $k = 'Total' unless defined $k; 
   $k = 'Unknown' if $k eq '';
   $pivot{$k} = $v;
}

[khandielas]

It turns out the limit for the concat string is set by mySQL.

After set group_concat_max_len to a big number, the query works fine.

$sth = $dbh->do('SET @@group_concat_max_len = 320000')

Thank you all!

[Anonymous Monk]

Obligatory :) bobby-tables.com: A guide to preventing SQL injection

[Anonymous Monk]

Why are your SQL statements so complex, while you seem to perform a rather normal task?