分组查询的简单问题，求解~~~实在没分了

[xyz_obj]

    select count(*) as C_Count, u.fullname ,c.seat_jobnumber,DATEDIFF(ss,call_time,offhook_time) as diff_time from tb_call as c left join tb_user as u on u.username = c.seat_jobnumber where 1=1  and c.call_time >='2012-02-09' and c.call_time <'2012-02-09 23:59:59' group by c.seat_jobnumber,u.fullname,call_time

会报错：“列 'c.offhook_time' 在选择列表中无效，因为该列既不包含在聚合函数中，也不包含在 GROUP BY 子句中。”
要怎么改才能达到我想要的这个结果呢？

[勿勿]

因为该列既不包含在聚合函数中，也不包含在 GROUP BY 子句中

[勿勿]

select count(*) as C_Count, u.fullname ,c.seat_jobnumber,DATEDIFF(ss,call_time,offhook_time) as diff_time from tb_call as c left join tb_user as u on u.username = c.seat_jobnumber where 1=1  and c.call_time >='2012-02-09' and c.call_time <'2012-02-09 23:59:59'

[xyz_obj]

[Quote=引用 2 楼 szstephenzhou 的回复:]
SQL code
select count(*) as C_Count, u.fullname ,c.seat_jobnumber,DATEDIFF(ss,call_time,offhook_time) as diff_time from tb_call as c left join tb_user as u on u.username = c.seat_jobnumber where 1=1 ……
[/Quote]
我要的是分组后的结果