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 报错 ORA-00937: 非单组分组函数.
------解决方法--------------------------------------------------------
SELECT DW AS 单位,XB AS 性别,SUM(COUNT) AS 人数 FROM stu_table
group by dw xb;