Self Join:
Self Join is nothing but joining one table with different names. We can use Inner Join or Outer Join to join the table.
Here is the small example which gives the difference between consecutive days of temperature.
CREATE TABLE #TBL (CDATE DATETIME , TEMP INT)
INSERT INTO #TBL
VALUES( ’2011-11-30 16:11:59.797′ , 20),
(’2011-12-01 16:11:59.797′ , 30),
(’2011-12-02 16:11:59.797′ , 10),
(’2011-12-03 16:11:59.797′ , 50),
(’2011-12-04 16:11:59.797′ , 20),
(’2011-12-05 16:11:59.797′ , 70),
(’2011-12-06 16:11:59.797′ , 40),
(’2011-12-07 16:11:59.797′ , 30),
(’2011-12-08 16:11:59.797′ , 10),
(’2011-12-09 16:11:59.797′ , 50)
SELECT CONVERT(VARCHAR(30) , T.CDATE,6) + ‘———’ + CONVERT(VARCHAR(30) , TT.CDATE,6) CURRENT_DT
,T.TEMP CURRENT_DATE_TEMP , TT.TEMP TOMORROW_TEMP
, T.TEMP – TT.TEMP DIFF
FROM #TBL T
INNER JOIN #TBL TT ON TT.CDATE = DATEADD(DD,1,T.CDATE )
DROP TABLE #TBL
