Codeigniter。 如何获取数据库列的总和

Question

I have two tables .

pre_order

receive_pre_order

My Question is how to get all the information from pre_order table and get the SUM of all the quantity received form receive_pre_order table against that pre_order id .
To make it clear suppose we have the following data in recieve_pre_order .

+--------------+------------+-------------------+
| pre_order_id | product_id | quantity_recieved |
+--------------+------------+-------------------+
|            1 |          1 |                10 | 
|            1 |          1 |                20 | 
|            1 |          1 |                10 |
+--------------+------------+-------------------+

It should return quantity_recieved = 40 against pre_order_id 1 .

Right now I have tried:

$this->db->select_sum('quantity_recieved');
$this->db->from('recieve_pre_order');
return $this->db->get();

It is working but, it is getting data from one table.
But I want to fetch data from both tables.

Answer 1

Join the two tables tables ( rpo.pre_order_id = po.id ). Group the result for po.id , so all Rows with the same po.id will be grouped into one row. sum(rpo.quantity_recieved) will sum up the quantity for these groups.

select po.id, sum(rpo.quantity_recieved)
from receive_pre_order rpo 
join pre_order po on rpo.pre_order_id = po.id  
group by po.id; 

Answer 2

try this

  $this->db->select("sum(recieve_pre_order.quantity_recieved),pre_order.id");
  $this->db->from('recieve_pre_order');
  $this->db->join("pre_order",'recieve_pre_order.pre_order_id = pre_order.id','left outer');
  $this->db->where('recieve_pre_order.pre_order_id',your parameter);

Answer 3

  $this->db->select('pre_order.*');
  $this->db->select_sum('recieve_pre_order.quantity_recieved');
  $this->db->from('pre_order');
  $this->db->join("recieve_pre_order",'recieve_pre_order.pre_order_id = pre_order.id','left');

Answer 4

try sub query instead of joining table.

  $this->db->select("po.*,SELECT(SUM(quantity_recieved) from receive_pre_order where pre_order_id = po.id) as total_recieved_quantity",FALSE);
  $this->db->from('pre_order as po');
  return $this->db->get();