oracle case when else 语法和用法

case when else 基本结构:
case n
  when 1 then Action1;
when 2 then Action2;
when 3 then Action3;
else        ActionOther;
end;
示例一：
SELECT col1, col2,  
       CASE  
          WHEN col3 > 1 AND col3 <2  
             THEN '1'  
          WHEN col3 > 2 AND col3 <3  
             THEN '2'  
          WHEN col3 > 3 AND col3 <4  
             THEN '3'  
          ELSE '4'  
       END mylevel  
FROM table_51xit
case when else语法要点说明如下：
1、以CASE开头，以END结尾
2、分支中WHEN 后跟条件，THEN为显示结果
3、ELSE 为除此之外的默认情况，类似于高级语言程序中switch case的default，可以不加
4、END 后跟别名
示例：更复杂的示例代码：
select d.districtcode,(case when (substr(d.districtcode,3,6) = '0000') then ('100000'||substr(d.districtcode,1,2))
                     when (substr(d.districtcode,5,6) = '00') then ('100000'||substr(d.districtcode,1,2)||'00'||substr(d.districtcode,3,2))
                     else  ('100000'||substr(d.districtcode,1,2)||'00'||substr(d.districtcode,3,2)||'00'||substr(d.districtcode,5,2))
                     end) unitcode From vcmdistrictcode d where
not exists
(
select 1 From vcmunitinfo u where u.unitdistrictcode=d.districtcode
and u.unitcode=(case when (substr(d.districtcode,3,6) = '0000') then ('100000'||substr(d.districtcode,1,2))
                     when (substr(d.districtcode,5,6) = '00') then ('100000'||substr(d.districtcode,1,2)||'00'||substr(d.districtcode,3,2))
                     else  ('100000'||substr(d.districtcode,1,2)||'00'||substr(d.districtcode,3,2)||'00'||substr(d.districtcode,5,2))
                     end)
)
转自：http://tanw.iteye.com/blog/701023