MySQL分组求结果未全部输出

问题遇到的现象和发生背景

在表中，按学历和性别统计平均年龄，当有一个学历的有一个性别没有人时，没人的结果不输出。

问题相关代码，请勿粘贴截图

SELECT education,gender,AVG(year(now())-year(date_of_birth)) AS 平均年龄 FROM tbl_employees GROUP BY education UNION ALL SELECT education,gender,AVG(year(now())-year(date_of_birth)) AS 平均年龄 FROM tbl_employees GROUP BY gender ORDER BY 平均年龄;

运行结果及报错内容

能运行，但显示结果错误。

我的解答思路和尝试过的方法

我尝试了用union all输出全部结果，但还是不对。

我想要达到的结果

让全部结果输出
求指导！

SELECT education,gender,AVG(year(now())-year(date_of_birth)) AS 平均年龄 
FROM tbl_employees GROUP BY education,gender

select的非聚合函数字段，都要放到group by 后面

where gender is not null 去除没有值的

SELECT education,gender,AVG(year(now())-year(date_of_birth)) AS 平均年龄 
FROM tbl_employees where gender is not null GROUP BY education,gender