Think\Db\Query::buildSql PHP Method

    public function buildSql($sub = true)
    {
        return $sub ? '( ' . $this->select(false) . ' )' : $this->select(false);
    }

 protected function parseClosure($call, $show = true)
 {
     $query = new Query($this->connection);
     call_user_func_array($call, [&$query]);
     return $query->buildSql($show);
 }