我的问题是，我得到的是所有3个表中都存在的数据。但是我想显示其中任何一个表中都不存在的数据（即空格，或者如果该项在任何一个表中都不存在，则在该项上打印0）…我搜索了一下，发现可以通过fullouter连接来完成，但我不知道如何使用它。。控制器代码：

public function St_statement(){

        $startdate = $this->input->post('SDate');
        $enddate = $this->input->post('EDate');
        $date = str_replace('/', '-', $startdate);
        $newDate = date("Y-m-d", strtotime($date));
        $date2 = str_replace('/', '-', $enddate);
        $newDate2 = date("Y-m-d", strtotime($date2));
        $data['startdate'] = $startdate;
        $data['enddate'] = $enddate;
        if ($this->input->post('all'))
        {
        $this->db->where('billdate >=', $newDate);
        $this->db->where('billdate <=', $newDate2);
        $this->db->where('billdte >=', $newDate);
        $this->db->where('billdte <=', $newDate2);
        $this->db->select("Item");
        $this->db->select("pgroup");
        $this->db->select_sum("Stock");
        $this->db->select_sum("quantity");
        $this->db->select_sum("Qty");
        $this->db->from('opstock');
        $this->db->group_by("Item");
        $this->db->order_by("pgroup",'asc');

        $this->db->join('purchaseitem',' purchaseitem.Prdtname = opstock.Item','OUTER');
         $this->db->join('salesitem','salesitem.Prdtname =  purchaseitem.Prdtname','OUTER');
          $this->db->join('itemmaster','itemmaster.itemname =  purchaseitem.Prdtname','OUTER');
         $this->db->join('pgroup','pgroup.pgroupid = itemmaster.catcode','OUTER');

        $query = $this->db->get();
  $data['query'] = $query;

        $this->load->view('Inventory/St_Stmt', $data);
        //$this->load->view('Inventory/St_Stmt1', $data);
    }
    if($this->input->post('selected'))
    {
        if($name = $this->input->post('businessType'))
        {
        $this->db->where('billdate >=', $newDate);
        $this->db->where('billdate <=', $newDate2);
        $this->db->where('billdte >=', $newDate);
        $this->db->where('billdte <=', $newDate2);
        $this->db->where('pgroup',$name);
        $this->db->select("Item");
        $this->db->select("pgroup");
        $this->db->select_sum("Stock");
        $this->db->select_sum("quantity");
        $this->db->select_sum("Qty");
        $this->db->from('opstock');
        $this->db->group_by("Item");

        $this->db->join('purchaseitem',' purchaseitem.Prdtname = opstock.Item','OUTER');
         $this->db->join('salesitem','salesitem.Prdtname =  purchaseitem.Prdtname','OUTER');
          $this->db->join('itemmaster','itemmaster.itemname =  purchaseitem.Prdtname','OUTER');
         $this->db->join('pgroup','pgroup.pgroupid = itemmaster.catcode','OUTER');

        $query = $this->db->get();
  $data['query'] = $query;

        $this->load->view('Inventory/St_Stmt', $data);

    }

}
}