Question

我有以下情况（下表），我想根据它们是否为NULL来选择“X”或“Y”。

  X         Y            pick
  null     not null        Y
  not null not null        X
  not null null            X

包含数据'X'和'Y'或UNION ALLed的行如下所示：

  select 'X' as a
  union all
  select 'Y' as a

所以我尝试了以下SQL，但不确定“rownum＆lt; = 1”部分。仅当UNION ALL保留时，这将起作用（对于X和Y都不为空的情况）我查询两行的顺序。

  select a from 
  (
  select 'X' as a
  union all
  select 'Y' as a
  ) where a is not null and rownum<=1;

  select a from 
  (
  select null as a
  union all
  select 'Y' as a
  ) where a is not null and rownum<=1;

  select a from 
  (
  select 'X' as a
  union all
  select null as a
  ) where a is not null and rownum<=1;

以上是正确的方法吗？任何见解都将非常感激

Answer 1

聚结

http://download.oracle.com/docs/cd/B28359_01/server.111/b28286/functions023.htm

Answer 2

如果您想在SQL中使用特定订单，则必须要求它。你可以扩展你的联盟：

  select a from (
    select a,rownum as rn from 
    (
       select 'X' as a, 0 as Priority from dual
       union all
       select 'Y' as a, 1 as Priority from dual
    ) where a is not null order by Priority
  ) where rn = 1

虽然我承认我对甲骨文不太好。我相信rownum在WHERE子句中做了奇怪的事情，这就是为什么我引入了额外级别的查询。

Oracle UNION ALL保存查询中的记录顺序

2 个答案: