SQL连接和条件比较值不正确

我在做这些陈述，通过得到正确的最小值和最大值范围，我离开了

SELECT 
C.CourseId, 
C.GradeValue MinGradeValue,
isnull(min(m.GradeValue)-1,100) MAxGradeValue,
L.LetterGrade ,
L.[Description] 
    FROM CourseGrade C

    LEFT JOIN LetterGrades L
    ON C.LetterGradedId=L.LetterGradeId 

    LEFT JOIN CourseGrade m
    ON m.LetterGradedId=L.LetterGradeId  AND m.GradeValue > C.GradeValue

    GROUP BY
    C.CourseId, 
    C.GradeValue, L.LetterGrade ,
    L.[Description];

这就是正确的范围应该是什么样子：

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您的最后一次加入需要在CourseID上：

SELECT 
    C.CourseId, 
    C.GradeValue MinGradeValue,
    isnull(min(m.GradeValue)-1,100) MAxGradeValue,
    L.LetterGrade ,
    L.[Description] 
FROM CourseGrade C

    LEFT JOIN LetterGrades L
        ON C.LetterGradedId=L.LetterGradeId 

    LEFT JOIN CourseGrade m
        ON m.CourseId=c.CourseId
        AND m.GradeValue > C.GradeValue

GROUP BY
    C.CourseId, 
    C.GradeValue, L.LetterGrade ,
    L.[Description];

看

  LEFT JOIN CourseGrade m
    ON m.LetterGradedId=L.LetterGradeId  AND m.GradeValue > C.GradeValue

第一串 c、 等级值=0 这个条件是错误的

m.GradeValue > C.GradeValue

因此，您在min（m.GradeValue）中得到null，您的isnull返回100