Postgres - 根据列值添加或减少行

Question

我有一张桌子shipments：

--------------------------
id | event    | quantity |
--------------------------
1  | received | 20       |
2  | sent     | 30       | 
3  | received | 45       |

我想要一个查询，它将所有数量相加received，并从sent数量中减去该值。查询应返回结果35。

SELECT SUM(quantity)
FROM shipments
WHERE event = 'received';

该查询返回65. 我怎样才能让它也减去数量sent（并得到结果35）？

Answer 1

使用CASE根据 放置符号的表达式event。

SELECT sum(CASE event
             WHEN 'received' THEN
               quantity
             WHEN 'sent' THEN
               -quantity
             ELSE
               0
           END)
       FROM shipments;