请教多表查询结果按条件求和？

SQL code

select a.piaolei,a.renshu,b.mingcheng as pl from Tbl_checkIn a,tbl_PiaoLei bwhere a.piaolei=b.bianhao and convert(char(10),CheckInTime,120) >= '2012-08-16' and convert(char(10),CheckInTime,120) <= '2012-08-16' and CheckInNo='01' 

想把查询出来的结果根据条件b.mingcheng='北京'、b.mingcheng='上海',求出a.renshu的总和各是多少，请问应该怎么写？谢谢！

[解决办法]

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 convert(char(10),CheckInTime,120) >= '2012-08-16' and convert(char(10),CheckInTime,120) <= '2012-08-16' and CheckInNo='01'