Question

我有一个学生表学生（LRN，fName，lName，levelID）和一个成绩表GRADE（subjectID，年级，levelID，LRN，TimeAdded）我想向所有学生展示所有在线成绩的学生。我厌倦了这个查询，但我得到了这个结果。

Query: SELECT student.LRN, student.fName, student.lName, 
(SELECT grade.grade WHERE grade.subjectID = 'k2arts') AS 'k2arts',
(SELECT grade.grade WHERE grade.subjectID = 'k2math') AS 'k2math',
(SELECT grade.grade WHERE grade.subjectID = 'k2eng') AS 'k2eng',
(SELECT grade.grade WHERE grade.subjectID = 'k2valed') AS 'k2valed',
(SELECT grade.grade WHERE grade.subjectID = 'k2fil') AS 'k2fil' 
from student RIGHT JOIN grade on grade.LRN=student.LRN

结果： Query Result

Answer 1

您希望在每个子查询中建立STUDENT的关系。您不需要在主查询中加入Grade，因为您不使用任何值，除非通过子查询，在这种情况下处于不相关的上下文中：

SELECT student.LRN, student.fName, student.lName, 
(SELECT grade.grade FROM grade WHERE grade.subjectID = 'k2arts' AND grade.LRN=st.LRN) AS 'k2arts',
(SELECT grade.grade FROM grade WHERE grade.subjectID = 'k2math' AND grade.LRN=st.LRN) AS 'k2math',
(SELECT grade.grade FROM grade WHERE grade.subjectID = 'k2eng' AND grade.LRN=st.LRN) AS 'k2eng',
(SELECT grade.grade FROM grade WHERE grade.subjectID = 'k2valed' AND grade.LRN=st.LRN) AS 'k2valed',
(SELECT grade.grade FROM grade WHERE grade.subjectID = 'k2fil' AND grade.LRN=st.LRN) AS 'k2fil' 
from student st

Answer 2

您可以使用左连接和

时的情况

SELECT student.LRN, student.fName, student.lName, 
  case when grade.subjectID = 'k2arts' then grade.grade else null end AS 'k2arts',
  case when grade.subjectID = 'k2math' then grade.grade else null end AS 'k2math',
  case when grade.subjectID = 'k2eng' then grade.grade else null end AS 'k2eng',
  case when grade.subjectID = 'k2valed' then grade.grade else null end AS 'k2valed',
  case when grade.subjectID = 'k2fil' then grade.grade else null end AS 'k2fil'
from student 
LEFT JOIN grade on grade.LRN=student.LRN

并最终使用伪聚合函数删除null

SELECT student.LRN, student.fName, student.lName, 
  min(case when grade.subjectID = 'k2arts' then grade.grade else null end) AS 'k2arts',
  min(case when grade.subjectID = 'k2math' then grade.grade else null end) AS 'k2math',
  min(case when grade.subjectID = 'k2eng' then grade.grade else null end) AS 'k2eng',
  min(case when grade.subjectID = 'k2valed' then grade.grade else null end) AS 'k2valed',
  min(case when grade.subjectID = 'k2fil' then grade.grade else null end) AS 'k2fil'
from student  student.LRN, student.fName, student.lName
LEFT JOIN grade on grade.LRN=student.LRN 
group by

Answer 3

尝试：

SELECT student.LRN, student.fName, student.lName, 
CASE WHEN grade.subjectID = 'k2arts' THEN grade.grade END AS 'k2arts',
CASE WHEN grade.subjectID = 'k2math'  THEN grade.grade END AS 'k2math',
CASE WHEN grade.subjectID = 'k2eng'   THEN grade.grade END AS 'k2eng',
CASE WHEN grade.subjectID = 'k2valed' THEN grade.grade END AS 'k2valed',
CASE WHEN grade.subjectID = 'k2fil'   THEN grade.grade END AS 'k2fil' 
from student 
LEFT JOIN grade 
on grade.LRN=student.LRN ;

查询获取学生和成绩SQL

3 个答案: