Question

下面的两个SQL查询应该给我完全相同的结果，但由于某种原因，第一个使用连接而不是子查询的查询给了我更多的结果。唯一的区别是两个样本中的最后一行。谁能告诉我为什么？

SELECT *
FROM TravelReport TR
  JOIN School SC ON SC.SchoolId = TR.SchoolId 
  LEFT OUTER JOIN [User] U ON U.UserId = TR.UserId
                          AND U.UserTypeId = 3 
                          AND U.UserStatusId = 1 
  JOIN ProjectSchools PS ON PS.SchoolId = SC.SchoolId
WHERE PS.Active = 1
  AND PS.ProjectId = 2

=====

SELECT *
FROM TravelReport TR
  JOIN School SC ON SC.SchoolId = TR.SchoolId 
  LEFT OUTER JOIN [User] U ON U.UserId = TR.UserId
                          AND U.UserTypeId = 3 
                          AND U.UserStatusId = 1 
WHERE SC.SchoolId in (SELECT DISTINCT PS.SchoolId FROM ProjectSchools PS
                      WHERE PS.Active = 1 AND PS.ProjectId = 2)

Answer 1

可以消除左边因为它不应该影响任何计数你确定你不只是得到更多相同的结果吗？

SELECT *
  FROM TravelReport TR      
  JOIN School SC 
        ON SC.SchoolId = TR.SchoolId  
  JOIN ProjectSchools PS 
        ON PS.SchoolId = SC.SchoolId 
 WHERE PS.Active    = 1
   AND PS.ProjectId = 2
=====

SELECT *
  FROM TravelReport TR
  JOIN School SC 
        ON SC.SchoolId = TR.SchoolId  
 WHERE SC.SchoolId in ( SELECT DISTINCT PS.SchoolId 
                          FROM ProjectSchools PS
                         WHERE PS.Active    = 1 
                           AND PS.ProjectId = 2 )

考虑一下

SchoolID  Active  ProjectID 
10        1       2
10        1       2 
10        1       1
11        2       2
12        1       2
13        1       2
13        2       2

第一个将为schoolID 10返回两行，第二个不会

也许学校可以拥有多个活跃的项目，因此您无法使SchoolID独一无二至少在SchoolID，Active，ProjedtID

上设置了一个独特的约束

或者只是在select *
中加上一个明显的选择不同的*

如果还有其他事情发生，请发布实际数据以重现问题

SQL查询应该做同样的事情，但不要

1 个答案: