sql复杂查询语法的mysql性能如何优化？

SELECT
       (CASE
         WHEN 
          (SELECT COUNT(*)
             FROM EMPLOYEE
            WHERE PERCOD = DB.PERID) > 1 THEN
          (SELECT PERNAME
             FROM EMPLOYEE
            WHERE PERCOD = DB.PERID
             AND ISDELETED = 0)
         ELSE
          (SELECT PERNAME
             FROM EMPLOYEE
            WHERE PERCOD = DB.PERID)
       END) AS PERNAME
  FROM BALANCE DB
  LEFT JOIN BALANCE_BONUS DBB
  ON DBB.ID = DB.BONUSID
 WHERE  DB.ISDELETED != 1
 ORDER BY DB.ID; 

----------------------------------------------------

(CASE
         WHEN 
          (SELECT COUNT(*)
             FROM EMPLOYEE
            WHERE PERCOD = DB.PERID) > 1 THEN
          (SELECT PERNAME
             FROM EMPLOYEE
            WHERE PERCOD = DB.PERID
             AND ISDELETED = 0)
         ELSE
          (SELECT PERNAME
             FROM EMPLOYEE
            WHERE PERCOD = DB.PERID)
       END) AS PERNAME

这一段代码会非常消耗查询时间。 作用是 排除 EMPLOYEE 表中PERCOD重复的数据 ， ISDELETED字段为已删除。 首先查询级联的人员是否有重复数据，如果只有一条则显示这唯一的一条； 如果有多条，则显示ISDELETED字段为0的一条数据。 有没有大牛给个优化解决方法！！！