[英]SQL Query with Sub-Query - Is there a better way?
for the 3 tables below 对于下面的3个表
Student 学生
StudentId, StudentName
1 aaa
2 bbb
3 ccc
Course 课程
CourseId, CourseName
100 xxx
101 yyy
102 zzz
StudentCourse StudentCourse
StudentId, CourseID
1 100
1 101
2 100
2 102
To select all the courses for the first student alphabetically that attends course xxx, I could write the following query: 要为第一个按字母顺序选择参加xxx课程的学生选择所有课程,我可以编写以下查询:
SELECT StudentName, CourseName
FROM Student
INNER JOIN StudentCourse ON Student.StudentId = StudentCourse.StudentID
INNER JOIN Course on StudentCourse.CourseID = Course.CourseID
WHERE StudentName in (
SELECT TOP 1 StudentName
FROM Student
INNER JOIN StudentCourse on Student.StudentID = StudentCourse.StudentID
INNER JOIN Course on StudentCourse.CourseID = Course.StudentID
WHERE CourseName='xxx'
ORDER BY StudentName)
Is there a more efficient way of doing/writing this without the sub-select, because it appears I am doing the same query twice. 有没有一种更有效的方式来执行/写入此操作而无需进行子选择,因为看来我在两次执行相同的查询。
Thanks. 谢谢。
You can use window functions and top (1) with ties
: 您可以将窗口函数和
top (1) with ties
:
SELECT TOP (1) WITH TIES StudentName, CourseName
FROM (SELECT s.StudentName, c.CourseName,
SUM(CASE WHEN c.CourseName = 'XXX' THEN 1 ELSE 0 END) OVER (PARTITION BY StudentName) as cnt_xxx
FROM Student s INNER JOIN
StudentCourse sc
ON s.StudentId = sc.StudentID INNER JOIN
Course c
ON sc.CourseID = c.CourseID
) sc
WHERE cnt_xxx > 0
ORDER BY StudentName;
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