public function setExpr($context, $expr, $args)
{
$prefix = '';
if ($this->first) {
$this->first = false;
$prefix = 'ON DUPLICATE KEY UPDATE ';
} else {
$prefix = ',';
}
$context->appendSql("{$prefix}{$expr}", $prefix == 'ON DUPLICATE KEY UPDATE ');
$context->appendParams($args);
}