新手Oracle数据库查询

要把第二个count的值除以两个count加起来的值，怎么写

SELECT DISTINCT AREANAME, SUM(SHMSL2)/(SUM(SHMSL1)+SUM(SHMSL2)) OVER(PARTITION BY AREANAME) AS SHMSL
  FROM (SELECT R.NAME 


               AREANAME,
                COUNT (CASE WHEN TS.SHMSL = '0' THEN 1     
                       ELSE NULL
                       END) SHMSL1 ,
                COUNT (CASE WHEN TS.SHMSL = '1' THEN 1
                       ELSE NULL
                       END) SHMSL2
          FROM T_XNJD_ITEM_SERVICES TS
          left join T_SYS_REGION R on R.AREAID = TS.AREAID
         GROUP BY R.NAME 


                 ,
                  TS.SHMSL)
 GROUP BY AREANAME,SHMSL