取自here的示例，我试图了解GROUPBY到底做了什么:给定这个employee表:+-------+----------+--------+------------+|Empid|Empname|Salary|DOB|+-------+----------+--------+------------+|1|Habib|2014|2004-12-02||2|Karan|4021|2003-04-11||3|Samia|22|2008-02-23||4|HuiLing|25|2008-10-15||5|Yumie|29|1999-01-26|+-------+----------+--