从每一列中选择最大值，即使其中一个值为空

Question

我有一张road_age_test桌子：

create table road_age_test (
    surface_year int,
    base_year int
);

insert into road_age_test (surface_year, base_year) values (10,20);
insert into road_age_test (surface_year, base_year) values (30,null);
insert into road_age_test (surface_year, base_year) values (null,40);
insert into road_age_test (surface_year, base_year) values (null,null);
insert into road_age_test (surface_year, base_year) values (50,50);

commit;

select * from road_age_test;

SURFACE_YEAR  BASE_YEAR
------------ ----------
          10         20
          30       null    
        null         40
        null       null
          50         50

我想从每一列中选择最大值，即使其中一个值为空：

GREATEST_YEAR
-------------
           20
           30
           40
         null
           50

什么是最简单的方法来做到这一点？

---

注意：如果两个值都是null，我想返回null，而不是0。

Answer 1

select
GREATEST(nvl(surface_year,0),nvl(base_year,0)) as greatest_year
from
road_age_test;

--Edit using NULLIF 
select
NULLIF(GREATEST(nvl(surface_year,0),nvl(base_year,0)),0) as greatest_year
from
road_age_test;

--Adding a segmented version for readability (some find it worse, eye of the beholder I guess)
select
NULLIF(
    GREATEST(
            nvl(surface_year,0),
            nvl(base_year,0) --Just keep adding columns here if you need to
            )
      ,0) as greatest_year
from
road_age_test;

无法想象它的效率要高得多（并且由于索引而不应在 where 子句中使用），但它比 case 语句更简洁，并允许您轻松添加更多值（列）。

https://docs.oracle.com/cd/B19306_01/server.102/b14200/functions060.htm