Drupal Field Handler

mymodule.module

<?php 
// Drupal does it the 'offical' sql way which can be a pain when you just want to do it the mysql way
// This is the workaround
// Tags are set in the view display Query setting
function mymodule_query_alter(QueryAlterableInterface $query) {
  $view_name = 'project_isotope';
  if ($query->hasTag('views_' . $view_name)) {
       $query->groupBy('node.title');
  }
}

mymodule.views.inc

<?php

/**
 * Implements hook_views_data().
 */
 function mymodule_views_data() {
   $data = array();
   $data['field_data_field_colour']['table']['group'] = t('Test colours');
   $data['field_data_field_colour']['table']['join'] = array(
     // #global is a special flag which let's a table appear all the time.

   );

   $data['field_data_field_colour']['get_colours'] = array(
     'title' => t('Test Handler'),
     'help' => t('Custom Handler.'),
     'field' => array(
       'handler' => 'mymodule_handler_field_colour_rgb',
     ),
   );

   return $data;
 }

mymodule_handler_field_colour_rgb.inc

<?php
/**
 * @file
 * Definition of mymodule_handler_handlername.
 */

/**
 * Description of what my handler does.
 */
class mymodule_handler_field_colour_rgb extends views_handler_field {
  /**
   * Add some required fields needed on render().
   */
  function construct() {
    parent::construct();
    $this->additional_fields['entity_id'] = array(
      'table' => 'field_data_field_colour',
      'field' => 'entity_id',
);
  }

  function query() {
    $this->ensure_my_table();
    $this->add_additional_fields();
    $this->field_alias = $this->table . '_' . $this->field;
    $this->query->add_field(NULL, "GROUP_CONCAT($this->table.field_colour_rgb)", $this->field_alias, array('aggregate' => TRUE));

        $table = $this->table; // i.e. how the table is represented in drupal database schema, without prefixes.
        $table_alias = 'field_data_field_colour';
        $left_table =  'field_data_field_category';// the table to which I want to join it. Mostly I assume you'll want to join it to the table of your handler
        $left_field = 'field_category_target_id'; // a column in $left_table on which you wanna join
        $field = 'entity_id'; // a column in $table on which you wanna join
        // We create a new join object and provide it with information about the join we wanna make.
        $extra = array();
        $extra[] = array(
          'table' => $table_alias,
          'field' => 'deleted',
          'value' => 0,
          'operator' => '=',
        );
        $type = 'LEFT';
        $join = new views_join();
        $join->construct($table, $left_table, $left_field, $field, $extra, $type);

         // Alias under which you wanna see $table once it has been JOINed with all the other tables.
        //add the join the the view query
        $this->query->add_relationship($table_alias, $join, $this->relationship);
      //  $this->query->add_groupby('node_title');
  }

  function render($values) {
    return format_string('color-' . $values->{$this->field_data_field_colour.entity_id},$values->{$this->field_alias});
  }

}