大侠请进，帮忙优化一条SQL语句
SQL code:
SELECT top 20 *,CONVERT(varchar(20) , HangUpDateTime, 20) AS HangUpDateTime,
CONVERT(varchar(20) , InboundDateTime, 20) AS InboundDateTime from InboundLog a
WHERE NOT EXISTS(
SELECT * from InboundLog b WHERE a.id<b.id AND b.InboundDateTime >= '2010-01-01 00:00:00' and b.InboundDateTime <= '2010-01-31 23:59:59'
AND a.callerID=b.callerID AND a.local_dial_number=b.local_dial_number
AND a.campaignScriptID=b.campaignScriptID AND
DATEDIFF(hh,a.inboundDateTime,b.inboundDateTime)<1 )
AND a.InboundDateTime >= '2010-01-01 00:00:00' and a.InboundDateTime <= '2010-01-31 23:59:59'
order by InboundDateTime desc
为什么加了a.id <b.id 这个查询条件，速度就慢很多，不加的话，就快。。
id有加索引的。
order by 很费时费力的
SQL code:
SELECT
top 20 *,
CONVERT(varchar(20) , A.HangUpDateTime, 20) AS HangUpDateTime,
CONVERT(varchar(20) , A.InboundDateTime, 20) AS InboundDateTime
from InboundLog a,
(SELECT
callerID,
local_dial_number,
campaignScriptID,
MAX(ID)BID
from
InboundLog b
WHERE DATEDIFF(MM, b.InboundDateTime ,'2010-01-01')=0
GROUP BY callerID, local_dial_number,campaignScriptID)AS B
WHERE
DATEDIFF(hh,a.inboundDateTime,b.inbound
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