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利用case when 减少表扫描次数

时间:2015-07-21 12:07:20      阅读:98      评论:0      收藏:0      [点我收藏+]

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原文:利用case when 减少表扫描次数

数据库环境:SQL SERVER 2008R2

有网友希望有人帮他优化一下他的SQL,SQL语句如下:

WITH T AS (
SELECT B.O_Money MON,B.O_States STATES FROM M_Basket A 
JOIN M_OrderInfo B ON A.OrderID=B.ID WHERE A.GoodID=@GOODSID ), B AS ( SELECT (SELECT SUM(MON) FROM T)SumMoney, (SELECT SUM(MON) FROM T WHERE STATES IN (2,3,4))ComfirmMoney, (SELECT COUNT(*) FROM T WHERE STATES=2)AleadyDrive, (SELECT COUNT(*) FROM T WHERE STATES=3)AleadyPay, (SELECT COUNT(*) FROM T WHERE STATES=4)AleadyComfirm ) INSERT @BIAO SELECT * FROM B

看了一下语句,有可能出问题的地方,是with B 里面的内容,T表被访问了4次,

稍微转换一下思路,用case when来改写这段,就可以让T表只访问1次。

改写的SQL如下:

WITH    T AS ( SELECT   B.O_Money MON ,
                        B.O_States STATES
               FROM     M_Basket A
                        JOIN M_OrderInfo B ON A.OrderID = B.ID
               WHERE    A.GoodID = @GOODSID
             ),
        B AS ( SELECT   SUM(MON) AS SumMoney ,
                        SUM(CASE WHEN STATES IN ( 2, 3, 4 ) THEN MON
                            END) AS ComfirmMoney ,
                        COUNT(CASE WHEN STATES = 2 THEN 1
                              END) AS AleadyDrive ,
                        COUNT(CASE WHEN STATES = 3 THEN 1
                              END) AS AleadyPay ,
                        COUNT(CASE WHEN STATES = 4 THEN 1
                              END) AS AleadyComfirm
               FROM     T
             )
    INSERT  @BIAO
            SELECT  *
            FROM    B

(本文完)

利用case when 减少表扫描次数

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原文地址:http://www.cnblogs.com/lonelyxmas/p/4663654.html

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