Question

我有以下查询。奇怪的是，它为同一个人返回了多条记录 - 但它应该为每个人返回一行。它是基于CONTACT1 C的LEFT JOINS - 每个人只有一行，不像其他列有时为同一个人有多行。

    select 
 C.ACCOUNTNO as 'AdmitGold Account',
 C2.UNAMEFIRST as 'First Name',
 C2.UNAMELAST as 'Last Name',
 C.KEY1 as 'Status',
 C.KEY4 as 'People ID',
 C.KEY3 as 'Type',
 C.KEY5 as 'Counselor',
 C.CITY as 'City',
 C.STATE as 'State',
 C.SOURCE as 'Source',
 C.DEPARTMENT as 'Major',
 C2.UGENDER as 'Gender',
 C2.UETHNICBG as 'Ethnicity',
 C2.UFULLPART as 'Full/Part',
 SLF_CLG_CS.EXT as 'College - GPA',
 OFF_CLG_CS.EXT as 'College - GPA Official',
 HS_OFF_CS.LINKACCT as 'HS GPA - Official',
 OFF_SAT_COMP.LINKACCT as 'SAT - Verbal',
 OFF_SAT_COMP.COUNTRY as 'SAT - Math',
 (Cast(OFF_SAT_COMP.LINKACCT as float) + Cast(OFF_SAT_COMP.COUNTRY as float)) as 'SAT - Composite',
 OFF_SAT_COMP.EXT as 'SAT - Essay',
 OFF_ACT_COMP.LINKACCT as 'ACT - English',
 OFF_ACT_COMP.COUNTRY as 'ACT - Math',
 OFF_ACT_COMP.ZIP as 'ACT - Reading',
 OFF_ACT_COMP.EXT as 'ACT - ScRe',
 (Cast(OFF_ACT_COMP.LINKACCT as float) + Cast(OFF_ACT_COMP.COUNTRY as float)+ Cast(OFF_ACT_COMP.ZIP as float) + Cast(OFF_ACT_COMP.EXT as float)) as 'ACT - Official' 
     from contact1 C
 left join CONTACT2 C2 on C.ACCOUNTNO=C2.ACCOUNTNO
 left join CONTSUPP HS_OFF_CS on C.ACCOUNTNO=HS_OFF_CS.ACCOUNTNO
  AND HS_OFF_CS.STATE='O' AND HS_OFF_CS.CONTACT='High School'
 left join CONTSUPP SLF_CLG_CS on C.ACCOUNTNO=SLF_CLG_CS.ACCOUNTNO
  AND SLF_CLG_CS.CONTACT = 'Transfer College' AND SLF_CLG_CS.STATE='S'
 left join CONTSUPP OFF_CLG_CS on C.ACCOUNTNO=OFF_CLG_CS.ACCOUNTNO
  AND OFF_CLG_CS.CONTACT = 'Transfer College' AND OFF_CLG_CS.STATE='O'
 left join CONTSUPP OFF_SAT_COMP on C.ACCOUNTNO=OFF_SAT_COMP.ACCOUNTNO
  AND OFF_SAT_COMP.CONTACT='Test/SAT' AND OFF_SAT_COMP.ZIP='O'
 left join CONTSUPP OFF_ACT_COMP on C.ACCOUNTNO=OFF_ACT_COMP.ACCOUNTNO
  AND OFF_ACT_COMP.CONTACT='Test/ACT' AND OFF_ACT_COMP.STATE='O'
     where 
 C.KEY1!='00PRSP' 
 AND C.U_KEY2='2010 FALL'

Answer 1

左连接将以1对多关系生成重复项。无论第一个表中有多少条记录，如果您将连接添加到第一个表中每个记录有多行的表中，您将获得多行。如果所有列的行实际都是重复的，则选择Distinct将删除重复项，但不会消除任何列中具有不同值的“重复项”。

Answer 2

如果您在服务器上显示计划权限，则可以快速识别重复项的来源 - 添加WHERE子句（例如，WHERE C.ACCOUNTNO ='某个值'），您希望将其返回单行（但是，如果您确定该值实际带回＆gt; 1行），请启用“包含实际执行计划”，运行查询并将鼠标悬停在计划各阶段之间的链接上 - 在某些时候您会发现＆GT; 1个记录来自特定的阶段，看这个阶段的细节可以揭示重复的原因。

为什么SQL查询返回重复？

2 个答案: