Postgres - 根据列值添加或减去行

Question

我有一张表shipments ：

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

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

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

此查询返回65 。 我怎样才能让它也减去sent的数量（并得到35的结果）？

Answer 1

使用根据event放置符号的CASE表达式。

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