Question

愚蠢的简单问题，我最终花了3.5个小时。我遇到了很多语法错误，所以如果有人能帮我解答这个问题，我会学到很多东西！非常感谢你！

我有3个数据库表：

Students Table
student_id, name
1, joe
2, jill

Courses Table
course_id, course_name
eng123, Engineering
stat111, Statistics

Marks Table
student_id, course_id, mark
1, stat111, 64
2, stat111, 90
1, eng123, 86

我需要编写一个SQL查询，它会给我一个总结的报告卡，如下所示：

student_id, student_name, eng123, stat 111
1, joe, 86, 64
2, jill, null, 90

---我探索的内容：我已经把PIVOT，CASE和GROUP BY作为我的主要领导，但我不能把最后的作品放在一起。到目前为止，我最有希望的查询是：

SELECT Students.student_id, Students.student_name, 
        CASE course_id WHEN 'eng123' THEN mark END as 'eng123',
        CASE course_id WHEN 'stat111' THEN mark END as 'stat111'
      FROM Students
      INNER JOIN Marks
      ON Students.student_id=Marks.student_id;

但这给了我不正确的结果：

student_id, student_name, eng123, stat111
1, joe, null, 64
1, joe, 86, null
2, jill, null, 90

Answer 1

您有希望的查询距离您想要的东西还有一箭之遥。您只需将其修改为GROUP BY student_id和student_name（应始终将它们配对在一起）。然后取标记的总和。请注意，我在ELSE语句中添加了CASE条件，这些条件赋值为0（因此不会影响总和）。

SELECT Students.student_id, Students.student_name, 
    SUM(CASE course_id WHEN 'eng123' THEN mark ELSE 0 END) as 'eng123',
    SUM(CASE course_id WHEN 'stat111' THEN mark ELSE 0 END) as 'stat111'
FROM Students
INNER JOIN Marks ON Students.student_id=Marks.student_id
GROUP BY Students.student_id, Students.student_name

Answer 2

      select student_id,student_name,
max(case when course_id = 'eng123' then mark end) as eng123,
max(case when course_id = 'stat111' then mark end) as stat111
from (
SELECT Students.student_id as student_id,
 Students.name as student_name,
 Marks.course_id as course_id,
 Marks.mark as mark
      FROM Student Students
      INNER JOIN Marks
      ON Students.student_id=Marks.student_id)Z
      group by student_id,student_name

Answer 3

您好尝试使用聚合函数MAX。试试这个：

WITH x AS (SELECT 1 AS student_id, 'joe' AS student_NAME FROM dual UNION ALL
                   SELECT 2 AS student_id, 'jill' AS student_NAME FROM dual),
         y AS (SELECT 1 AS student_id , 'stat111' AS course_id, 64 AS mark FROM dual UNION ALL
                   SELECT 2 AS student_id , 'stat111' AS course_id, 90 AS mark FROM dual UNION ALL
                   SELECT 1 AS student_id , 'eng123' AS course_id, 86 AS mark FROM dual      )

SELECT x.student_id, x.student_name, 
       MAX (CASE WHEN course_id = 'eng123' THEN mark END)  eng123,
       max(CASE WHEN course_id = 'stat111' THEN mark END)  stat111
      FROM x
      INNER JOIN y
      ON x.student_id=y.student_id
      group by x.student_id, x.student_name;

SQL Query，用于汇总学生在报告卡中的标记

3 个答案: