oracle sql中是否有一种方法可以通过交叉联接获得计数？

Question

我需要列出一个专业列表和一个字母等级列表，并且需要获得与该字母等级相关的专业内完成的课程数量。所以，如果我有：

Major
Mechanical Engineer
Electrical Engineer
Civil Engineer
Chemical Engineer 
...

Grade
A
B
C
...

Student
100 - Alice - Mechanical Engineer
101 - Tom - Mechanical Engineer
102 - Rex - Mechanical Engineer
103 - Bob - Mechanical Engineer
104 - John - Civil Engineer 
105 - Alex - Electrical Engineer

Course
001 - Solid Mechanics - 100 - A
002 - Thermodynamics - 100 - A
003 - Heat Transfer - 100 - A 
004 - Heat Transfer - 101 - A
005 - Gadgetry - 100 - A
006 - Gadgetry - 101 - A
007 - Gadgetry - 102 - A
008 - Dynamics - 102 - A
009 - Gadgetry - 101 - C
010 - Heat Transfer - 102 - C
011 - Fluid Mechanics - 100 - B 
012 - Materials - 102 -B 
013 - Intro to EE - 105 - B
014 - Embedded Systems - 105 - B
015 - Analog Circuits - 105 - B
... 

我需要输出：

Mechanical Engineer - A - 8
Mechanical Engineer - B - 2
Mechanical Engineer - C - 2
Electrical Engineer - A - 0
Electrical Engineer - B - 3
...

[还有一张学生用桌，说明每个学生所在的专业，课程表上该学生上哪门课程（有些学生不能上任何课程），以及从那门课程获得的分数。

到目前为止，我有：

SELECT major_name, grade, COUNT(grade_code) OVER (PARTITION BY major_name)
FROM Major CROSS JOIN ((grade LEFT JOIN course USING (grade_code)) RIGHT JOIN student USING (st_id))
ORDER BY major_name, grade;

但是该计数不起作用（每个专业都得到相同的不正确计数），实质上是课程表中字母等级的总计数。

Mechanical Engineer - A - 10
Mechanical Engineer - B - 10
Mechanical Engineer - C - 10
Electrical Engineer - A - 10
...

Answer 1

不需要CROSS JOIN或合计COUNT...OVER()窗口功能。只需联接主表（student和course），然后联接查找表（major和grade）。然后，合计计数。因为major链接到不在FROM子句中的student，所以用相应的别名替换传统USING的ON：

SELECT m.major_name, 
       g.grade, 
       count(*) as major_grade_count
FROM course c
LEFT JOIN student s USING (st_id)
LEFT JOIN grade g USING (grade_code)
LEFT JOIN major m ON g.major_code = m.major_code      -- ADJUST FIELD HERE
GROUP BY  m.major_name, 
          g.grade;