SELECT CONVERT(VARCHAR(7),时间,120) AS 日期
,SUM(CASE WHEN TYPE = 2 THEN 1 ELSE 0 END) AS 离职人数
,SUM(CASE WHEN TYPE = 1 THEN 1 ELSE 0 END) AS 入职人数
FROM 
SELECT 入职时间 AS 时间,1 AS TYPE FROM 表一 
UNION ALL
SELECT 离职时间 AS 时间,2 AS TYPE FROM 表二
) AS A
GROUP BY CONVERT(VARCHAR(7),时间,120)

------解决方案--------------------
SQL code


;with t1 as 
(
    select '2012-07-08' as t union all
    select '2012-06-09' as t 
),t2 as 
(
    select '2012-05-06' as t union all
    select '2012-06-04' as t 
),t3 as 
(
    select t,1 as s from t1 union all
    select t,2 from t2
)
select convert(varchar(7),t,120) as 日期, 
    sum(case when s = 2 then 1 else 0 end) as [离职人数],
    sum(case when s = 1 then 1 else 0 end) as [入职人数]
from t3 
group by convert(varchar(7),t,120)

/*
日期      离职人数        入职人数
------- ----------- -----------
2012-05 1           0
2012-06 1           1
2012-07 0           1

(3 行受影响)


*/

------解决方案--------------------
SQL code

----------------------------
-- Author  :TravyLee(努力工作中！！！)
-- Date    :2012-08-08 08:55:41
-- Version:
--      Microsoft SQL Server 2008 R2 (RTM) - 10.50.1600.1 (X64) 
--    Apr  2 2010 15:48:46 
--    Copyright (c) Microsoft Corporation
--    Enterprise Edition (64-bit) on Windows NT 6.1 <X64> (Build 7600: ) (Hypervisor)
--
----------------------------
--> 测试数据：[A]
if object_id('[A]') is not null drop table [A]
go 
create table [A]([InDate] datetime)
insert [A]
select '2012-07-08' union all
select '2012-06-09'
go
--> 测试数据：[B]
if object_id('[B]') is not null drop table [B]
go 
create table [B]([OutDate] datetime)
insert [B]
select '2012-05-06' union all
select '2012-06-04'
go

;with 
t as(
select *,'IN' as States from A
union all
select *,'Out' from B
)
select 
    convert(varchar(7),Indate,120) as 时间,
    SUM(case when States='IN' then 1 else 0 end) as 入职,
    SUM(case when States='Out' then 1 else 0 end) as 离职
from t
group by 
convert(varchar(7),Indate,120)

/*
时间    入职    离职
---------------------------------
2012-05    0    1
2012-06    1    1
2012-07    1    0
*/

------解决方案--------------------
SQL code

--> 测试数据：
declare @TA table ([INDATE] datetime)
declare @TB table ([OUTDATE] datetime)
insert @TA
select '2012-07-08' union all
select '2012-06-09'
insert @TB
select '2012-05-06' union all
select '2012-06-04'

;with cte as
(select INDATE,'IN' as States from @TA
union all
select OUTDATE,'OUT' as States from @TB
)
select 
    convert(varchar(7),INDATE,120) as 时间,
    SUM(case when States='IN' then 1 else 0 end) as 入职人数,
    SUM(case when States='OUT' then 1 else 0 end) as 离职人数
from cte
group by 
convert(varchar(7),INDATE,120)

/*
时间    入职    离职
---------------------------------
2012-05    0    1
2012-06    1    1
2012-07    1    0
*/