I have a query to return the most recent n posts per discussion (example below has n as 3).

    SELECT x.group_discussion_id, x.comment, x.date_posted, 
        x.username, x.realname,
        TO_DAYS(NOW()) - TO_DAYS(x.date_posted) AS date_posted_days
    FROM (
          SELECT gdp.group_discussion_post_id, 
              gdp.group_discussion_id, gdp.comment, 
              gdp.date_posted, m.username, m.realname, 
          @recent_post_rank := IF
                  (
                      @prev_discussion = gdp.group_discussion_id, 
                      @recent_post_rank + 1, 
                      1
                  ) AS recent_post_rank, 
          @prev_discussion := gdp.group_discussion_id
      FROM group_discussion_posts gdp 
          JOIN members m 
          ON m.memberid = gdp.memberid 
          WHERE gdp.active = 1 
          AND gdp.reply_to_post_id != 0
          AND gdp.group_discussion_id IN (1, 2, 3)  
      ORDER BY gdp.group_discussion_id DESC, 
              gdp.group_discussion_post_id DESC
          ) x
      WHERE recent_post_rank <= 3
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This query works perfectly in MySQL, I have also run the $sth->{Statement}; output which runs the same. However, running the query on my site, using DBI does not return the same output, it does not restrict the rows returned to the n value i.e. all of the matching group_discussion_posts are returned. Debugging this I found that dbi returns the recent_post_rank as 1 for every row (unlike when it is run directly in MySQL), and there is no value for @prev_discussion Therefore, it would appear that DBI is not properly handling this @prev_discussion stored value. Any ideas?

    my $sth = $dbh->prepare('SELECT x.group_discussion_id, x.comment, 
+x.date_posted, 
        x.username, x.realname,
        TO_DAYS(NOW()) - TO_DAYS(x.date_posted) AS date_posted_days
    FROM (
          SELECT gdp.group_discussion_post_id, 
              gdp.group_discussion_id, gdp.comment, 
              gdp.date_posted, m.username, m.realname, 
          @recent_post_rank := IF
                  (
                      @prev_discussion = gdp.group_discussion_id, 
                      @recent_post_rank + 1, 
                      1
                  ) AS recent_post_rank, 
          @prev_discussion := gdp.group_discussion_id
      FROM group_discussion_posts gdp 
          JOIN members m 
          ON m.memberid = gdp.memberid 
          WHERE gdp.active = 1 
          AND gdp.reply_to_post_id != 0
          AND gdp.group_discussion_id IN (1, 2, 3)  
      ORDER BY gdp.group_discussion_id DESC, 
              gdp.group_discussion_post_id DESC
          ) x
      WHERE recent_post_rank <= 3' );

    $sth->execute();
    return $sth->fetchall_arrayref();
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