with rollup用法：
1.没用with rollup:

SELECT name, SUM(signin) as signin_count 
 FROM  employee_tbl 
 GROUP BY name

2.用了with rollup:

SELECT name, SUM(signin) as signin_count 
 FROM  employee_tbl 
 GROUP BY name 
 WITH ROLLUP;

3.如果用了with rollup,想把“NULL”改成其他名字，可用coalesce：

参数说明：如果a为null,则选择b；如果b==null,则选择c；如果a!=null,则选择a；如果a b c 都为null ，则返回为null（没意义）。

SELECT coalesce(name, '总数'), SUM(signin) as signin_count 
FROM  employee_tbl 
GROUP BY name 
WITH ROLLUP;