报错 ORA-00937: 非单组分组函数,该如何处理

报错 ORA-00937: 非单组分组函数
SELECT   DW   AS   单位,XB   AS   性别,SUM(COUNT)   AS   人数   FROM   stu_table;

加了SUM(COUNT)   AS   人数就报错.   请教~

------解决方案--------------------
SELECT DW AS 单位,XB AS 性别,SUM(COUNT) AS 人数 FROM stu_table
group by dw xb;