根据条件t-sql连接表自身

Question

如果有人可以帮我查询，我会很感激。 我有一张包含每种货币Currencies和Amounts的表格。 我的第一个问题是：

select Currency,  SUM(Amount) amnt from [MyTable] 
where CODE in(410, 420) 
group by Currency

所以，我得到了：

Currency | amnt
USD      | 15

我想显示我的表格中的所有可用货币，并加入这个结果（这些货币的金额必须为空）

Currency | amnt
USD      | 15
EUR      | 
AED      |

我按查询获取所有货币的清单：

select currency from [MyTable] group by currency

那么，我如何加入这两个查询？

提前致谢！

Answer 1

试试这个：

SELECT a.Currency, SUM(CASE WHEN a.CODE IN (410, 420) THEN a.Amount ELSE 0 END) amnt 
FROM MyTable a 
GROUP BY a.Currency

或

SELECT A.Currency, SUM(B.Amount) amnt 
FROM (SELECT DISTINCT Currency FROM MyTable) A
LEFT JOIN MyTable B ON A.Currency = B.Currency AND B.CODE IN (410, 420)
GROUP BY A.Currency

或

SELECT A.Currency, SUM(B.Amount) amnt 
FROM (SELECT Currency FROM MyTable GROUP BY Currency) A
LEFT JOIN MyTable B ON A.Currency = B.Currency AND B.CODE IN (410, 420)
GROUP BY A.Currency