一张表查询，只取每个cust_id的前三条记录，求解！该怎么解决

--> 测试数据：[tbl]if object_id('[tbl]') is not null drop table [tbl]create table [tbl]([Id] int,[cust_id] int,[call_date] datetime,[call_result] varchar(7),[C5] varchar(5))insert [tbl]select 1,1,'2012-03-15 09:00:00','fail',null union allselect 2,1,'2012-03-15 09:05:00','number','error' union allselect 3,1,'2012-03-15 09:10:00','fail',null union allselect 4,1,'2012-03-15 09:15:00','success',null union allselect 5,2,'2012-03-15 09:01:00','fail',null union allselect 6,2,'2012-03-15 09:06:00','number','error' union allselect 7,2,'2012-03-15 09:17:00','fail',null union allselect 8,2,'2012-03-15 09:18:00','success',null union allselect 9,3,'2012-03-15 09:04:00','fail',null union allselect 10,3,'2012-03-15 09:09:00','number','error' union allselect 11,3,'2012-03-15 09:19:00','fail',null union allselect 12,3,'2012-03-15 09:22:22','success',null union allselect 13,4,'2012-03-15 09:07:00','fail',null union allselect 14,4,'2012-03-15 09:21:40','success',nullselect [Id] ,[cust_id],[call_date] ,[call_result] from(select ROW_NUMBER()OVER(partition by [cust_id] order by [call_date] desc) as num,* from tbl)a where num<=3/*Id    cust_id    call_date    call_result4    1    2012-03-15 09:15:00.000    success3    1    2012-03-15 09:10:00.000    fail2    1    2012-03-15 09:05:00.000    number8    2    2012-03-15 09:18:00.000    success7    2    2012-03-15 09:17:00.000    fail6    2    2012-03-15 09:06:00.000    number12    3    2012-03-15 09:22:22.000    success11    3    2012-03-15 09:19:00.000    fail10    3    2012-03-15 09:09:00.000    number14    4    2012-03-15 09:21:40.000    success13    4    2012-03-15 09:07:00.000    fail*/按时间取每个cus_id的前三