declare @T table (col datetime)
insert into @T
select '20010101' union all
select '20010402' union all
select '20010523' union all
select '20010623' union all
select '20010809' union all
select '20010911' union all
select '20011112' union all
select '20011214' union all
select '20020219' union all
select '20020326' union all
select '20020505' union all
select '20020611' union all
select '20020915' union all
select '20021230'

select a.col as c1,b.col as c2 from @T a 
right join (
select dateadd(month,number,'20010101') as col 
from master..spt_values where type='p' and number < 24) b
on convert(varchar(7),a.col,120)=convert(varchar(7),b.col,120)
/*
c1                      c2
----------------------- -----------------------
2001-01-01 00:00:00.000 2001-01-01 00:00:00.000
NULL                    2001-02-01 00:00:00.000
NULL                    2001-03-01 00:00:00.000
2001-04-02 00:00:00.000 2001-04-01 00:00:00.000
2001-05-23 00:00:00.000 2001-05-01 00:00:00.000
2001-06-23 00:00:00.000 2001-06-01 00:00:00.000
NULL                    2001-07-01 00:00:00.000
2001-08-09 00:00:00.000 2001-08-01 00:00:00.000
2001-09-11 00:00:00.000 2001-09-01 00:00:00.000
NULL                    2001-10-01 00:00:00.000
2001-11-12 00:00:00.000 2001-11-01 00:00:00.000
2001-12-14 00:00:00.000 2001-12-01 00:00:00.000
NULL                    2002-01-01 00:00:00.000
2002-02-19 00:00:00.000 2002-02-01 00:00:00.000
2002-03-26 00:00:00.000 2002-03-01 00:00:00.000
NULL                    2002-04-01 00:00:00.000
2002-05-05 00:00:00.000 2002-05-01 00:00:00.000
2002-06-11 00:00:00.000 2002-06-01 00:00:00.000
NULL                    2002-07-01 00:00:00.000
NULL                    2002-08-01 00:00:00.000
2002-09-15 00:00:00.000 2002-09-01 00:00:00.000
NULL                    2002-10-01 00:00:00.000
NULL                    2002-11-01 00:00:00.000
2002-12-30 00:00:00.000 2002-12-01 00:00:00.000
*/

------解决方案--------------------

SQL code
CREATE TABLE TABLE5(DocDate DATETIME)
INSERT INTO TABLE5
SELECT '20010101' UNION ALL
SELECT '20010402' UNION ALL
SELECT '20010523' UNION ALL
SELECT '20010623' UNION ALL
SELECT '20010809' UNION ALL
SELECT '20010911' UNION ALL
SELECT '20011112' UNION ALL
SELECT '20011214' UNION ALL
SELECT '20020219' UNION ALL
SELECT '20020326' UNION ALL
SELECT '20020505' UNION ALL
SELECT '20020611' UNION ALL
SELECT '20020915' UNION ALL
SELECT '20021230'  

DECLARE @MINMONTH DATETIME
DECLARE @MAXMONTH DATETIME
DECLARE @TMEPMONTH DATETIME
SELECT @MINMONTH =MIN(DocDate),@MAXMONTH =MAX(DocDate) FROM TABLE5  

CREATE TABLE #TABLE6(DocDate DATETIME)

WHILE DATEDIFF(MONTH,@MINMONTH,@MAXMONTH)>0
BEGIN
SET @MINMONTH =DATEADD(MONTH,1,@MINMONTH)
IF NOT EXISTS(SELECT 1 FROM TABLE5 WHERE CONVERT(CHAR(7),DocDate,111) = CONVERT(CHAR(7),@MINMONTH,111) )
BEGIN
INSERT INTO #TABLE6
SELECT @MINMONTH
END
END

SELECT * FROM #TABLE6

DROP TABLE #TABLE6

/*
DocDate
-----------------------
2001-02-01 00:00:00.000
2001-03-01 00:00:00.000
2001-07-01 00:00:00.000
2001-10-01 00:00:00.000
2002-01-01 00:00:00.000
2002-04-01 00:00:00.000
2002-07-01 00:00:00.000
2002-08-01 00:00:00.000
2002-10-01 00:00:00.000
2002-11-01 00:00:00.000

(10 行受影响)

*/