Question

select distinct
       dbo.Courses.DisplayName AS CourseInternalName,
       dbo.courses.CourseID, 
       stuff((select distinct ','+ (case when CourseSchedules.dayid = 1 then 'Mon'
                                         when CourseSchedules.dayid = 2 then 'Tue'
                                         when CourseSchedules.dayid = 3 then 'Wed' 
                                         when CourseSchedules.dayid = 4 then 'Thur'
                                         when CourseSchedules.dayid = 5 then 'Fri'
                                         when CourseSchedules.dayid = 6 then 'Sat'
                                         when CourseSchedules.dayid = 7 then 'Sun'
                                     end) 
               FROM  dbo.Courses 
               left outer join dbo.CourseSchedules 
                    on dbo.Courses.CourseGUID = dbo.CourseSchedules.CourseGUID
               for xml path('')),1,1,'') AS Days

from  dbo.Courses 
left outer join dbo.CourseSchedules 
     on dbo.Courses.CourseGUID = dbo.CourseSchedules.CourseGUID
Where courses.CourseID = 5815

结果是：

CourseInternalName  CourseID    Days
Red Cross Sunfish - CC  5815    Mon,Fri,Wed,Tue,Thur,Sat

这是一周中的所有日子，而不仅仅是课程运行的日子。

还需要订购天。

如果我在Stuff语句中删除了from子句，我会得到正确的日子，但是会有多行。

Answer 1

您的左连接是多余的，导致您的查询返回意外结果。

请改为尝试：

首先，创建并填充样本表（请在将来的问题中保存此步骤）：

CREATE TABLE Courses
(
    CourseID int,
    DisplayName varchar(10),
    CourseGUID uniqueidentifier DEFAULT(NEWID())
)

CREATE TABLE CourseSchedules
(
    dayid int,
    CourseGUID uniqueidentifier
)

INSERT INTO Courses(CourseID, DisplayName)
VALUES (5815, 'Test')

INSERT INTO CourseSchedules
SELECT 1, CourseGUID FROM Courses UNION
SELECT 3, CourseGUID FROM Courses UNION
SELECT 5, CourseGUID FROM Courses

查询：

SELECT DISTINCT
       dbo.Courses.DisplayName AS CourseInternalName,
       dbo.courses.CourseID, 
       STUFF((SELECT ','+ CASE WHEN dayid = 1 THEN 'Mon'
                               WHEN dayid = 2 THEN 'Tue'
                               WHEN dayid = 3 THEN 'Wed' 
                               WHEN dayid = 4 THEN 'Thur'
                               WHEN dayid = 5 THEN 'Fri'
                               WHEN dayid = 6 THEN 'Sat'
                               WHEN dayid = 7 THEN 'Sun'
                          END
               FROM  dbo.CourseSchedules 
               WHERE dbo.CourseSchedules.CourseGUID = dbo.Courses.CourseGUID
               ORDER BY dayid
               FOR XML PATH('')),1,1,'') AS Days
FROM  dbo.Courses 
WHERE courses.CourseID = 5815

结果：

CourseInternalName  CourseID    Days
Test                5815        Mon,Wed,Fri

Answer 2

问题来自你的子查询，你没有告诉它只选择一门课程，你要告诉它选择任何与课程相匹配的课程。修正如下。实际上，当子查询完全分离时，不需要第一次连接

select distinct
       dbo.Courses.DisplayName AS CourseInternalName,
       dbo.courses.CourseID, 
       stuff((select distinct ','+ (case when CourseSchedules.dayid = 1 then 'Mon'
                                         when CourseSchedules.dayid = 2 then 'Tue'
                                         when CourseSchedules.dayid = 3 then 'Wed' 
                                         when CourseSchedules.dayid = 4 then 'Thur'
                                         when CourseSchedules.dayid = 5 then 'Fri'
                                         when CourseSchedules.dayid = 6 then 'Sat'
                                         when CourseSchedules.dayid = 7 then 'Sun'
                                     end) 
               FROM  dbo.CourseSchedules
               Where CourseSchedules.CourseID = 5815
               ORDER BY CourseSchedules.dayid ASC
               for xml path('')),1,1,'') AS Days

from  dbo.Courses 
Where courses.CourseID = 5815

多行到一行。获取所有日子而不是那些匹配课程的日子。以下是结果

2 个答案: