Sql 当有空单元格时，如何连接列

嗨，我有一个表，其中有4列，我需要这是'sell_currency'，'sell_currency'，'buy_currency'和'buy_currency'.currency列是varchar，而amount是数字，我想要实现的是通过其货币计算金额的总和，所以这是我用来得到结果的SQL:

  select
    sell_currency,
    sum(sell_amount) as sell_amount
    null as buy_currency
    null as buy_amount
  from table1
  group by
    sell_currency
  union
  select
    null as sell_currency,
    null as sell_amount,
    buy_currency,
    sum(buy_amount) as buy_amount
  from table1
  group by
    buy_currency

使用表1中的数据:

SELL_CURRENCY   SELL_AMOUNT BUY_CURRENCY    BUY_AMOUNT
A               123         D               1
B               234         E               2
B               345         D               3
A               222         (null)          0
C               4444        (null)          0

这是我从表中得到的结果:

SELL_CURRENCY   SELL_AMOUNT BUY_CURRENCY    BUY_AMOUNT
A               345         (null)          (null)
B               579         (null)          (null)
C               4444        (null)          (null)  
(null)          (null)      D               4
(null)          (null)      E               2

但是，我想删除这些null，并能够将行组合在一起，以实现如下所示的结果:

SELL_CURRENCY   SELL_AMOUNT BUY_CURRENCY    BUY_AMOUNT
A               345         D               4
B               579         E               2
C               4444        (null)          (null)  

我需要做什么才能达到这个结果？

为了了解更多信息，这4列也经常更新，因此在卖出和买入中可能有3或2种以上的独特货币.