Howto use sql formula in views field

Hi

I want to implement such code

SELECT node.nid AS nid, somesqlfun(mytable.field1, mytable.field2) AS myfield
FROM node node
INNER JOIN mytable ON node.nid=mytable.nid

for this I use such php-code

function mymodule_views_data() {
  $data['mymodule']['table']['group'] = t('My Module');
  $data['mymodule']['table']['join']['node'] = array(
    'type' => 'INNER',
    'left_table'  => 'node',
    'left_field'  => 'nid',
    'table'  => 'mytable',
    'field' => 'nid',
  );
  $data['mymodule']['myfield'] = array(
    'real field' => 'somesqlfun(mytable.field1, mytable.field2)',
    'title' => t('My  field'),
    'help'  => t('My field description'),
    'field' => array(
      'handler'   =>  'views_handler_field',
      'click sortable' => true,
    ),
  );
}

as result I get wrong SQL query

SELECT node.nid AS nid, mytable.somesqlfun(mytable.field1, mytable.field2) AS myfield
FROM node node
INNER JOIN mytable ON node.nid=mytable.nid

How I must change $data array in hook_views_data() to correct?