加入Select sql

Question

我想显示Purchase_Request的所有计数器

表Purchase_Order

counter | qty |      
---------------         
100001  | 10  |  
100001  | 10  |  
100001  | 10  |  
100004  | 30  |  

表Purchase_Request

counter | total_qty |
---------------------
100001  |     50    |  
100002  |     100   |  
100003  |     50    |  
100004  |     70    | 

我的代码只显示100001和100004，我想显示purchase_request中的所有计数器

    select a.counter,
           a.total_qty,
           a.total_qty - b.qty balance 
      from (select counter,
                   sum(total_qty) total_qty
              from purchase_request
          group by counter) a 
inner join (select counter,
                   sum(qty) qty
              from purchase_order 
          group by counter) b 
        on a.counter= b.counter 
  group by a.counter

帮助？