--> 测试数据：[入职表]
IF OBJECT_ID('[入职表]') IS NOT NULL DROP TABLE [入职表]
GO 
CREATE TABLE [入职表]([入职日期] DATETIME)
INSERT [入职表]
SELECT '2012-07-08' UNION ALL
SELECT '2012-07-12'
--------------开始查询--------------------------


--> 测试数据：[离职表]
IF OBJECT_ID('[离职表]') IS NOT NULL DROP TABLE [离职表]
GO 
CREATE TABLE [离职表]([离职日期] DATETIME)
INSERT [离职表]
SELECT '2012-07-06' UNION ALL
SELECT '2012-06-01'
--------------开始查询--------------------------

SELECT DISTINCT YEAR(ISNULL(a.[入职日期],b.[离职日期])) AS 年,MONTH(ISNULL(a.[入职日期],b.[离职日期])) AS 月,
(select count(*) FROM [入职表]  WHERE  CONVERT(VARCHAR(7),[入职日期],120)=CONVERT(VARCHAR(7),a.[入职日期],120)) AS 入职人数 ,
(select count(*) FROM [离职表]  WHERE  CONVERT(VARCHAR(7),[离职日期],120)=CONVERT(VARCHAR(7),b.[离职日期],120)) AS 离职人数 
FROM [入职表] AS a
FULL JOIN [离职表] AS b
ON  CONVERT(VARCHAR(7),a.[入职日期],120)=CONVERT(VARCHAR(7),b.[离职日期],120)
----------------结果----------------------------
/* 
年           月           入职人数        离职人数
----------- ----------- ----------- -----------
2012        6           0           1
2012        7           2           1

(2 行受影响)


*/

------解决方案--------------------
SQL code
select datepart(year,t.[日期]) '年份',datepart(month,t.[日期]) '月份',t.[入职人数],t.[离职人数] from 
(
    select (case when ([入职日期] IS NOT NULL AND [离职日期] IS NOT NULL) or ([入职日期] IS NOT NULL AND [离职日期] IS NULL) THEN [入职日期]
           case when [入职日期] IS NULL AND [离职日期] IS NOT NULL THEN [离职日期] END)  '日期',ISNULL([入职人数],0) '入职人数',ISNULL([离职人数],0) '离职人数'
    from 
    (
        select convert(varchar(7),[入职日期],120) '入职日期',sum(convert(varchar(7),[入职日期],120)) '入职人数' 
        from [入职表] 
        GROUP BY convert(varchar(7),[入职日期],120)
    ) a
    full join 
    (
        select convert(varchar(7),[离职日期],120) '离职日期',sum(convert(varchar(7),[离职日期],120)) '离职人数' 
        from [离职表] GROUP BY convert(varchar(7),[离职日期],120))
    ) b
    on a.[入职日期]=b.[离职日期]
) t