sql分组求和解决思路

sql分组求和
如题：
表结构：[entid] varchar，[smstotal] int，[smscharge] int，[smsentertime] datetime
要根据entid分组求smstotal，smscharge这两列的和，sql语句是这样的：
select entid,sum(smstotal),sum(smscharge) from qmsg_addsmsmmslog group by entid
但是我要限定时间（smsentertime）要怎么写啊？

------解决方案--------------------
select entid,sum(smstotal),sum(smscharge) from qmsg_addsmsmmslog where (时间的条件) group by entid
------解决方案--------------------
select entid,sum(smstotal),sum(smscharge) from qmsg_addsmsmmslog where smsentertime = ... group by entid

select entid,sum(smstotal),sum(smscharge) from qmsg_addsmsmmslog where smsentertime between ... and ... group by entid
------解决方案--------------------

SQL code

select entid,sum(smstotal),sum(smscharge) from qmsg_addsmsmmslog where smsentertime >=? --加个wheregroup by entid
------解决方案--------------------
加个时间字段的条件