Question

嘿伙计们，我有一个SQL语句，可以获取不同活动类型（家庭作业，测验等）的等级，如果该类型的分数最低，则会下降，否则，它仍然存在。错误在下面和SQL代码一样。

SELECT     Student.firstName, Student.lastName, 'Grades' =
           CASE 
              WHEN Grades.activityType = 'Homework' THEN
                CASE WHEN Policy.drop_hw = 1 THEN
                    (AVG(SUM(Grades.grade) - MIN(Grades.grade))) * (Policy.homework / 100)
                ELSE
                    (AVG(Grades.grade) * (Policy.homework / 100))
                END
            END,  Course.courseNum, Course.sectNum, Grades.activityType

FROM ...

以下是我遇到的错误：

- Cannot perform an aggregate function on an expression containing an aggregate or a subquery.
- Column 'Policy.drop_hw' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

Answer 1

研究分析功能。（SO question，Oracle documentation）。

这样的事情：

AVG(Grades.grade) OVER (PARTITION BY Grades.student_id) AS avg_of_grades

和

(AVG(SUM(Grades.grade) - MIN(Grades.grade))) OVER (PARTITION BY Grades.student_id) AS avg_grades_with_drop

在您的情况下使用任何有意义的内容设置分区;我们无法分辨，因为您在示例中省略了FROM ...。

然后，您可以在CASE语句中的任何计算中使用这些列别名。

Answer 2

如果你只需要降低一个最低等级（如果有关系）

SELECT  student_id, AVG(grade)
FROM    (
        SELECT  *, ROW_NUMBER() OVER (PARTITION BY student_id ORDER BY grade) rn
        FROM    my_tables
        )
WHERE   NOT (drop_hw = 1 AND rn = 1)
GROUP BY
        student_id

如果你需要放弃所有最低等级：

SELECT  student_id, AVG(grade)
FROM    (
        SELECT  *, MIN(grade) OVER (PARTITION BY student_id) mingrade
        FROM    my_tables
        )
WHERE   NOT (drop_hw = 1 AND grade = mingrade)
GROUP BY
        student_id

Answer 3

sum-operator给出一个结果（每组）。 min-operator也是。那么avg-operator聚合应该是什么？

总和减去最小值

3 个答案: