请教多表查询结果按日期求和？

SQL code

select sum(case when b.mingcheng='北京' then a.renshu else 0 end) '北京人数',sum(case when b.mingcheng='上海' then a.renshu else 0 end) '上海人数'from Tbl_checkIn a,tbl_PiaoLei bwhere a.piaolei=b.bianhao and CheckInTime>= '2012-10-01' and CheckInTime<= '2012-10-07' and CheckInNo='01' 

请教各位老师，这个是查询几天的合计人数，如何把查询出来的结果每天显示北京多少人，上海多少人呢？请问应该怎么写？谢谢！

[解决办法]

SQL code

select CheckInTime,sum(case when b.mingcheng='北京' then a.renshu else 0 end) '北京人数',sum(case when b.mingcheng='上海' then a.renshu else 0 end) '上海人数' from Tbl_checkIn a,tbl_PiaoLei b where a.piaolei=b.bianhao  and CheckInTime>= '2012-10-01'  and CheckInTime<= '2012-10-07'  and CheckInNo='01'  GROUP BY CheckInTime