Set up the query for this argument.
The argument sent may be found at $this->argument.
Overrides ArgumentPluginBase::query
public function query($group_by = FALSE) {
$this
->ensureMyTable();
$field = $this
->getField();
$placeholder = $this
->placeholder();
$this->query
->add_having_expression(0, "{$field} = {$placeholder}", array(
$placeholder => $this->argument,
));
}