数据库查询的 Case.......When的嵌套用法

数据库查询的重要条件判断：Case....When......用法

select E.Userid,
       CASE
         WHEN E.USERNAME IS NULL THEN
          '0'
         else
          E.USERNAME
       END USERNAME,
       CASE
         WHEN E.USERNAME IS NULL THEN
          '0'
         ELSE
          '1'
       END COMPARE
  from eep.users E