左连接表而不影响计算左列表？

Question

我在查找表格时遇到问题，我使用postgresql作为我的数据库

我有2张表A和表B如下

table A
---------------------------------------------------------------
|   headerid    |   amount  |   depart      |   arrival     |
---------------------------------------------------------------
|   100         |   10      |   australia   |   german      |
|   101         |   30      |   german      |   australia   |
|   102         |   25      |   australia   |   german      |
|   103         |   20      |   irak        |   iran        |

table B 
---------------------
| headerid |   name |
---------------------
|   100    |   a    |
|   100    |   b    |
|   101    |   c    |
|   102    |   d    |
|   103    |   e    |

我需要的结果是

---

|   depart      |   arrival     |   total amount|   name    |
-------------------------------------------------------------
|    australia  |   german      |   35          |       d   |       
|    german     |   australia   |   30          |       c   |        
|    irak       |   iran        |   20          |       e   |       

我到目前为止已经尝试过查询

http://sqlfiddle.com/#!12/6c20c/8

Answer 1

我真的不了解底层逻辑，但这个查询会产生你想要的东西：

SELECT depart, arrival, sum(amount) "total amount" , max(b1.name) "name" FROM a
JOIN (
  SELECT headerid, max(name) "name" FROM b
  GROUP BY headerid
) b1
ON a.headerid = b1.headerid
GROUP BY depart, arrival
ORDER BY depart

输出：

|    DEPART |   ARRIVAL | TOTAL AMOUNT | NAME |
|-----------|-----------|--------------|------|
| Australia |    German |           35 |    d |
|    German | Australia |           30 |    c |
|      irak |      iran |           20 |    e |

小提琴here。