SELECT
    (
        CASE
            WHEN 3<R<=10
            THEN 'A'
            WHEN 10<R<=20
            THEN 'B'
            WHEN 20<R<=30
            THEN 'C'
            WHEN R>30
            THEN 'D'
            ELSE '小于等于3的情况值'
        END ) AS R
FROM
    TABLE

补充: 

一楼SQL有个很明显的BUG~会把"小于等于3"和"大于30"的值全部赋值为D...题意是"当R>30时,才为D"

select (case when 3<R<=10 then  'A'

             when 10<R<=20 then 'B'

             when 20<R<=30 then 'C'

              else 'D' end ) as R from table

SELECT
        CASE
            WHEN R>3 AND R<=10 
            THEN 'A'
            WHEN R>10 AND R<=20
            THEN 'B'
            WHEN R>20 AND R<=30
            THEN 'C'
            WHEN R>30
            THEN 'D'
            ELSE '其他'
        END AS R
FROM
    TABLE