Question

我正在尝试构建一个SQL Union Query，它将返回两列（每个select一个），其中包含来自同一字段的结果，并使用WHERE子句进行排序。

以下是我的尝试：

SELECT scg.grade AS course_1
FROM cp_StudentCourseGrade AS scg
JOIN cp_CourseSecSem AS css ON
css.courseSecSem_id = scg.courseSecSem_id
JOIN cp_Course AS c1 ON
c1.course_id = css.course_id
UNION
SELECT scg.grade AS course_2
FROM cp_StudentCourseGrade AS scg
JOIN cp_CourseSecSem AS css ON
css.courseSecSem_id = scg.courseSecSem_id
JOIN cp_Course AS c2 ON
c2.course_id = css.course_id**
WHERE c1.course_id = c2.course_id AND c1.course_id=1 AND c2.course_id=2;

这应该给我这样的东西：

    course_1 | course_2
    -------------------
    4.0      | 6.0
    4.5      | 6.0
    6.0      | 5.5
    2.5      | 5.0
    1.0      | 4.0
    5.5      | 4.5

但我收到的错误是：

未知栏c1.course_id

有没有人知道我应该怎么做？

Answer 1

这就是你需要的：

SELECT scg1.grade AS course_1, scg2.grade AS course_2
FROM cp_StudentCourseGrade AS scg1
JOIN cp_CourseSecSem AS css1 ON
css1.courseSecSem_id = scg1.courseSecSem_id
JOIN cp_Course AS c1 ON
c1.course_id = css1.course_id AND c1.course_id=1 
JOIN cp_StudentCourseGrade AS scg2
JOIN cp_CourseSecSem AS css2 ON
css2.courseSecSem_id = scg2.courseSecSem_id
JOIN cp_Course AS c2 ON
c2.course_id = css2.course_id AND c2.course_id=2;

Answer 2

如果成绩应该与同一个学生相关联，我会猜到这样的事情：

Select
  scg.student_id,
  Sum(Case When css.course_id = 1 Then scg.grade Else Null End) as course_1,
  Sum(Case When css.course_id = 2 Then scg.grade Else Null End) as course_2
From
  cp_StudentCourseGrade As scg
    Inner Join 
  cp_CourseSecSem As css 
    On css.courseSecSem_id = scg.courseSecSem_id
Group By
  scg.student_id

SQL UNION查询在同一个表上，相同的字段

2 个答案: