如何回显SQL列表的SUM值

Question

我有这个SQL列表：

我有兴趣回复列表product_quantity的总量，所以它说301

//total
$this->tbl_inventory('product_quantity');
$data['total_quantity'] = count($this->global_model->get());

Answer 1

$this->tbl_inventory('product_quantity');
$data['total_quantity'] = sum($this->global_model->get());