sql带条件查找最小缺失编号

/**
     * select
     * case when not exists(select * from rt_test_case where CASE_NUM=1) then 1
     * else (select min(IFNULL(CASE_NUM,0)+1)  from rt_test_case  as a
     * where
     * not exists(
     * select CASE_NUM
     * from rt_test_case as b
     * where IFNULL(a.CASE_NUM,0)+1=b.CASE_NUM AND a.CASE_SET_ID = b.CASE_SET_ID
     * ) and CASE_SET_ID = '10383') end
     * */