这段SQL语句要怎么精简一些？

[wwooo]

 CREATE TABLE [dbo].[a] (
	[department] [nvarchar] (10) COLLATE Chinese_PRC_CI_AS NULL ,
	[employee] [nvarchar] (10) COLLATE Chinese_PRC_CI_AS NULL ,
	[month] [nvarchar] (10) COLLATE Chinese_PRC_CI_AS NULL ,
	[purchase_id] [nvarchar] (10) COLLATE Chinese_PRC_CI_AS NULL)
-----------------------------------------------------------------------
 INSERT [a] ( [department] , [employee] , [month] , [purchase_id] ) VALUES ( '1' , '001' , '01' , '0001' )
 INSERT [a] ( [department] , [employee] , [month] , [purchase_id] ) VALUES ( '1' , '002' , '01' , '0002' )
 INSERT [a] ( [department] , [employee] , [month] , [purchase_id] ) VALUES ( '1' , '002' , '01' , '0003' )
 INSERT [a] ( [department] , [employee] , [month] , [purchase_id] ) VALUES ( '1' , '002' , '02' , '0004' )
 INSERT [a] ( [department] , [employee] , [month] , [purchase_id] ) VALUES ( '1' , '001' , '02' , '0005' )
 INSERT [a] ( [department] , [employee] , [month] , [purchase_id] ) VALUES ( '2' , '003' , '01' , '0006' )
 INSERT [a] ( [department] , [employee] , [month] , [purchase_id] ) VALUES ( '2' , '003' , '01' , '0007' )
 INSERT [a] ( [department] , [employee] , [month] , [purchase_id] ) VALUES ( '2' , '003' , '01' , '0008' )
 INSERT [a] ( [department] , [employee] , [month] , [purchase_id] ) VALUES ( '2' , '003' , '02' , '0009' )
 INSERT [a] ( [department] , [employee] , [month] , [purchase_id] ) VALUES ( '2' , '004' , '02' , '0010' )
 --------------------------------------------
 select a.num,b.num_of_dep,a.num/b.num_of_dep as rate from
(SELECT   department,month,employee,count(*) as num
from a
group by department,month,employee) as a
join
(SELECT   department,month,count(*) as num_of_dep
from a
group by department,month) as b
on a.department=b.department and a.month=b.month

结果如下：
1 3 0
2 3 0
1 2 0
1 2 0
3 3 1
1 2 0
1 2 0


问题有两个：
1 语句里面有两个子查询，不知如何精简？
2 rate要怎么设定是decimal（5，2）的数据类型

[zheninchangjiang]

1-group by 不同，其他方法并无效率
2-cast(a.num/(b.num_of_dep*1.0) as numeric(5,2))