a表存在而b表不存在的关联数据

用for循环效率太低，采用以下sql

<select id="selectNotin" resultType="com.gaiaworks.pjo.entity.CdAdmin">
        select * from cd_admin a  where a.type=2 and(select count(1) FROM Cust_dept5th c where c.cust_headOfUnit =a.operate_num)=0
    </select>