sql-server - 根据另一列中的值选择值，其中一个为 NULL

Question

我有下表设置

Acc  Currency  Alias
1    NULL      A
1    USD       B
1    EUR       C

我想通过将输入作为Acc. 和连接到另一个表Currency。Acc

它应该按如下方式工作 -

If acc = 1 and currency is USD then B, if EUR then C, if null then A

有人可以帮忙吗？

Answer 1

你真的尝试过自己编写任何SQL 吗？

SELECT ...
FROM otherTable ot
(INNER) JOIN thisTable tt
ON ot.Acc = tt.Acc AND ot.Currency = tt.Currency 
...

Answer 2

显式匹配 NULL

SELECT *
  FROM othertbl o
  JOIN thistbl t on o.acc = t.acc
                and (o.currency = t.currency or o.currency is null AND t.currency is null)

或者在您的情况下，使用 NULL 货币thistbl作为后备

 SELECT ...
   FROM othertbl o
   JOIN thistbl t on o.acc = t.acc and o.currency = t.currency
  UNION ALL
 SELECT ...
   FROM othertbl o
   JOIN thistbl t on o.acc = t.acc and t.currency IS NULL
  WHERE NOT EXISTS (select *
                      from thistbl t2
                     where o.acc = t2.acc and o.currency = t2.currency)