Question

我想创建一个返回所有客户的查询，但只有在2008年2月放置时才匹配订单。这需要连接表Customers和Orders。

我使用了两个查询并得到了不同的答案。第二个是对的，但为什么呢？从逻辑上讲，我期待第一个是正确的，但事实并非如此。

SELECT  Cust.custid, Cust.contactname, Ord.orderid, Ord.orderdate 
FROM Sales.Customers AS Cust LEFT OUTER JOIN
Sales.Orders Ord
ON Cust.custid = Ord.custid
WHERE Ord.orderdate >= '2008-02-01' 
AND Ord.orderdate < '2008-03-01' 
order by Cust.custid, Ord.orderdate asc

第二次查询 -

SELECT C.custid, C.companyname, O.orderid, O.orderdate
FROM Sales.Customers AS C
LEFT OUTER JOIN Sales.Orders AS O
ON C.custid = O.custid
AND O.orderdate >= '20080201'
AND O.orderdate < '20080301';

下面添加了查询1和2的结果集。将AND部分放在Q1中的WHERE括号中没有任何效果。

Q1 -

custid,companyname,orderid,orderdate
4,Arndt, Torsten,10864,2008-02-02 00:00:00.000
5,Higginbotham, Tom,10866,2008-02-03 00:00:00.000
5,Higginbotham, Tom,10875,2008-02-06 00:00:00.000
9,Raghav, Amritansh,10871,2008-02-05 00:00:00.000
9,Raghav, Amritansh,10876,2008-02-09 00:00:00.000
12,Ray, Mike,10881,2008-02-11 00:00:00.000
18,Rizaldy, Arif,10890,2008-02-16 00:00:00.000
20,Kane, John,10895,2008-02-18 00:00:00.000
24,San Juan, Patricia,10880,2008-02-10 00:00:00.000
24,San Juan, Patricia,10902,2008-02-23 00:00:00.000
29,Kolesnikova, Katerina,10887,2008-02-13 00:00:00.000
30,Shabalin, Rostislav,10872,2008-02-05 00:00:00.000
30,Shabalin, Rostislav,10874,2008-02-06 00:00:00.000
30,Shabalin, Rostislav,10888,2008-02-16 00:00:00.000
30,Shabalin, Rostislav,10911,2008-02-26 00:00:00.000
34,Cohen, Shy,10886,2008-02-13 00:00:00.000
34,Cohen, Shy,10903,2008-02-24 00:00:00.000
35,Langohr, Kris,10863,2008-02-02 00:00:00.000
35,Langohr, Kris,10901,2008-02-23 00:00:00.000
37,Crăciun, Ovidiu V.,10897,2008-02-19 00:00:00.000
37,Crăciun, Ovidiu V.,10912,2008-02-26 00:00:00.000
39,Song, Lolan,10893,2008-02-18 00:00:00.000
44,Louverdis, George,10891,2008-02-17 00:00:00.000
45,Sunkammurali,  Krishna,10884,2008-02-12 00:00:00.000
46,Dressler, Marlies,10899,2008-02-20 00:00:00.000
48,Szymczak, Radosław,10867,2008-02-03 00:00:00.000
48,Szymczak, Radosław,10883,2008-02-12 00:00:00.000
50,Mace, Donald,10892,2008-02-17 00:00:00.000
50,Mace, Donald,10896,2008-02-19 00:00:00.000
54,Tiano, Mike,10898,2008-02-20 00:00:00.000
62,Misiec, Anna,10868,2008-02-04 00:00:00.000
62,Misiec, Anna,10913,2008-02-26 00:00:00.000
62,Misiec, Anna,10914,2008-02-27 00:00:00.000
63,Veronesi, Giorgio,10865,2008-02-02 00:00:00.000
63,Veronesi, Giorgio,10878,2008-02-10 00:00:00.000
64,Gaffney, Lawrie,10916,2008-02-27 00:00:00.000
65,Moore, Michael,10889,2008-02-16 00:00:00.000
66,Voss, Florian,10908,2008-02-26 00:00:00.000
67,Garden, Euan,10877,2008-02-09 00:00:00.000
70,Ginters, Kaspars,10909,2008-02-26 00:00:00.000
71,Navarro, Tomás,10882,2008-02-11 00:00:00.000
71,Navarro, Tomás,10894,2008-02-18 00:00:00.000
72,Welcker, Brian,10869,2008-02-04 00:00:00.000
74,O’Brien, Dave,10907,2008-02-25 00:00:00.000
76,Gulbis, Katrin,10885,2008-02-12 00:00:00.000
80,Geschwandtner, Jens,10915,2008-02-27 00:00:00.000
88,Li, Yan,10900,2008-02-20 00:00:00.000
88,Li, Yan,10905,2008-02-24 00:00:00.000
89,Smith Jr., Ronaldo,10904,2008-02-24 00:00:00.000
90,Larsson, Katarina,10873,2008-02-06 00:00:00.000
90,Larsson, Katarina,10879,2008-02-10 00:00:00.000
90,Larsson, Katarina,10910,2008-02-26 00:00:00.000
91,Conn, Steve,10870,2008-02-04 00:00:00.000
91,Conn, Steve,10906,2008-02-25 00:00:00.000

Q2 -

custid,companyname,orderid,orderdate
1,Customer NRZBB,NULL,NULL
2,Customer MLTDN,NULL,NULL
3,Customer KBUDE,NULL,NULL
4,Customer HFBZG,10864,2008-02-02 00:00:00.000
5,Customer HGVLZ,10866,2008-02-03 00:00:00.000
5,Customer HGVLZ,10875,2008-02-06 00:00:00.000
6,Customer XHXJV,NULL,NULL
7,Customer QXVLA,NULL,NULL
8,Customer QUHWH,NULL,NULL
9,Customer RTXGC,10876,2008-02-09 00:00:00.000
9,Customer RTXGC,10871,2008-02-05 00:00:00.000
10,Customer EEALV,NULL,NULL
11,Customer UBHAU,NULL,NULL
12,Customer PSNMQ,10881,2008-02-11 00:00:00.000
13,Customer VMLOG,NULL,NULL
14,Customer WNMAF,NULL,NULL
15,Customer JUWXK,NULL,NULL
16,Customer GYBBY,NULL,NULL
17,Customer FEVNN,NULL,NULL
18,Customer BSVAR,10890,2008-02-16 00:00:00.000
19,Customer RFNQC,NULL,NULL
20,Customer THHDP,10895,2008-02-18 00:00:00.000
21,Customer KIDPX,NULL,NULL
22,Customer DTDMN,NULL,NULL
23,Customer WVFAF,NULL,NULL
24,Customer CYZTN,10880,2008-02-10 00:00:00.000
24,Customer CYZTN,10902,2008-02-23 00:00:00.000
25,Customer AZJED,NULL,NULL
26,Customer USDBG,NULL,NULL
27,Customer WMFEA,NULL,NULL
28,Customer XYUFB,NULL,NULL
29,Customer MDLWA,10887,2008-02-13 00:00:00.000
30,Customer KSLQF,10888,2008-02-16 00:00:00.000
30,Customer KSLQF,10874,2008-02-06 00:00:00.000
30,Customer KSLQF,10872,2008-02-05 00:00:00.000
30,Customer KSLQF,10911,2008-02-26 00:00:00.000
31,Customer YJCBX,NULL,NULL
32,Customer YSIQX,NULL,NULL
33,Customer FVXPQ,NULL,NULL
34,Customer IBVRG,10903,2008-02-24 00:00:00.000
34,Customer IBVRG,10886,2008-02-13 00:00:00.000
35,Customer UMTLM,10901,2008-02-23 00:00:00.000
35,Customer UMTLM,10863,2008-02-02 00:00:00.000
36,Customer LVJSO,NULL,NULL
37,Customer FRXZL,10897,2008-02-19 00:00:00.000
37,Customer FRXZL,10912,2008-02-26 00:00:00.000
38,Customer LJUCA,NULL,NULL
39,Customer GLLAG,10893,2008-02-18 00:00:00.000
40,Customer EFFTC,NULL,NULL
41,Customer XIIWM,NULL,NULL
42,Customer IAIJK,NULL,NULL
43,Customer UISOJ,NULL,NULL
44,Customer OXFRU,10891,2008-02-17 00:00:00.000
45,Customer QXPPT,10884,2008-02-12 00:00:00.000
46,Customer XPNIK,10899,2008-02-20 00:00:00.000
47,Customer PSQUZ,NULL,NULL
48,Customer DVFMB,10883,2008-02-12 00:00:00.000
48,Customer DVFMB,10867,2008-02-03 00:00:00.000
49,Customer CQRAA,NULL,NULL
50,Customer JYPSC,10892,2008-02-17 00:00:00.000
50,Customer JYPSC,10896,2008-02-19 00:00:00.000
51,Customer PVDZC,NULL,NULL
52,Customer PZNLA,NULL,NULL
53,Customer GCJSG,NULL,NULL
54,Customer TDKEG,10898,2008-02-20 00:00:00.000
55,Customer KZQZT,NULL,NULL
56,Customer QNIVZ,NULL,NULL
57,Customer WVAXS,NULL,NULL
58,Customer AHXHT,NULL,NULL
59,Customer LOLJO,NULL,NULL
60,Customer QZURI,NULL,NULL
61,Customer WULWD,NULL,NULL
62,Customer WFIZJ,10868,2008-02-04 00:00:00.000
62,Customer WFIZJ,10913,2008-02-26 00:00:00.000
62,Customer WFIZJ,10914,2008-02-27 00:00:00.000
63,Customer IRRVL,10865,2008-02-02 00:00:00.000
63,Customer IRRVL,10878,2008-02-10 00:00:00.000
64,Customer LWGMD,10916,2008-02-27 00:00:00.000
65,Customer NYUHS,10889,2008-02-16 00:00:00.000
66,Customer LHANT,10908,2008-02-26 00:00:00.000
67,Customer QVEPD,10877,2008-02-09 00:00:00.000
68,Customer CCKOT,NULL,NULL
69,Customer SIUIH,NULL,NULL
70,Customer TMXGN,10909,2008-02-26 00:00:00.000
71,Customer LCOUJ,10882,2008-02-11 00:00:00.000
71,Customer LCOUJ,10894,2008-02-18 00:00:00.000
72,Customer AHPOP,10869,2008-02-04 00:00:00.000
73,Customer JMIKW,NULL,NULL
74,Customer YSHXL,10907,2008-02-25 00:00:00.000
75,Customer XOJYP,NULL,NULL
76,Customer SFOGW,10885,2008-02-12 00:00:00.000
77,Customer LCYBZ,NULL,NULL
78,Customer NLTYP,NULL,NULL
79,Customer FAPSM,NULL,NULL
80,Customer VONTK,10915,2008-02-27 00:00:00.000
81,Customer YQQWW,NULL,NULL
82,Customer EYHKM,NULL,NULL
83,Customer ZRNDE,NULL,NULL
84,Customer NRCSK,NULL,NULL
85,Customer ENQZT,NULL,NULL
86,Customer SNXOJ,NULL,NULL
87,Customer ZHYOS,NULL,NULL
88,Customer SRQVM,10905,2008-02-24 00:00:00.000
88,Customer SRQVM,10900,2008-02-20 00:00:00.000
89,Customer YBQTI,10904,2008-02-24 00:00:00.000
90,Customer XBBVR,10910,2008-02-26 00:00:00.000
90,Customer XBBVR,10873,2008-02-06 00:00:00.000
90,Customer XBBVR,10879,2008-02-10 00:00:00.000
91,Customer CCFIZ,10870,2008-02-04 00:00:00.000
91,Customer CCFIZ,10906,2008-02-25 00:00:00.000

Answer 1

第一个查询将首先加入Customers和Orders，然后它将根据您的WHERE子句过滤掉所有行。也就是说，在该日期没有记录的客户（对于他们，orderdate等将为null）范围将被排除在外。

第二个查询将返回所有客户，但它只会加入订单日期在指定范围内的订单记录。

我希望这是有道理的 - 如果您需要进一步澄清，请告诉我！

Answer 2

在from子句之前评估where子句。

在第一种情况下，如果订单在范围内没有日期，则结果将从查询中过滤掉。这意味着客户没有要包含在输出中的行。

在第二种情况下，如果订单在范围内没有日期，则它会使on子句失败。该查询仍在执行left join，因此它会保留customer行。第二个表中的值为NULL。没有其他过滤，因此客户记录显示在输出中。

Answer 3

当有外连接时，where子句应用于外连接的结果。因此差异。在第一个查询条件是where子句的一部分，这意味着一旦所有客户被检索到where子句应用于给定范围内的订单，那么它将会遗漏一些没有任何订单的客户。

在第二次查询的情况下，无论订单是否存在，都会报告所有客户。如果存在，将检查订单范围日期。因此第二个查询是正确的。

使用JOIN时使用WHERE过滤？

3 个答案: