合并多个记录到一个基于一些条件

Question

我有这样如下表：

payee  | payer | amount 
-----------|-----------|---------- 
USER3  | USER1 | 150.00 
USER3  | USER2 | 50.00 
USER1  | USER3 | 300.00 
USER2  | USER3 | 300.00 
USER3  | USER6 | 100.00 

我想两个记录，其中收款人是第二个记录和付款人的付款相结合是第二记录的收款人和重新计算的金额。 所以我需要的结果是：

payee  | payer  | amount 
---------- | ------------| --------- 
USER1  | USER3  | 150.00 
USER2  | USER3  | 250.00 
USER3  | USER6  | 100.00 

Answer 1

大多数数据库least()和greatest()功能，所以一个方法是：

select least(payee, payer) as payee, greatest(payee, payer) as payer, 
     sum(case when payee = least(payee, payer) then amount else - amount end) as amount 
from t 
group by least(payee, payer), greatest(payee, payer); 

（在那些没有可以使用case表达。）

如果你总是想要正数，你可以使用子查询：

select (case when amount < 0 then payer else payee end) as payee, 
     (case when amount < 0 then payee else payer end) as payer, 
     abs(amount) 
from (select least(payee, payer) as payee, greatest(payee, payer) as payer, 
      sum(case when payee = least(payee, payer) then amount else - amount end) as amount 
     from t 
     group by least(payee, payer), greatest(payee, payer) 
    ) t;