create view table that calculate total values base on multiple column values - mysql

Question

These are my example table
course table include
course_id | course name
1 | java
2 | .net
3 | php
4 | ruby and rails
Indian_student table include
course_id | no.of student
2 | 10
3 | 30
Japan_student table include
course_id | no.of student
1 | 50
2 | 30
Chinese_student table include
course_id | no.of student
2 | 60
4 | 20
I want the output as
Course_id | in_stu | ja_stu | ch_stu | total
1 | 0 | 50 | 0 | 50
2 | 10 | 30 | 60 | 100
3 | 30 | 0 | 0 | 30
4 | 0 | 0 | 20 |20
But I only get the result
Course_id | in_stu | ja_stu | ch_stu | total
2 | 10 | 30 | 60 | 100
my view is
create view total_student as select
i.indian_stu as in_stu,
j.japan_stu as ja_stu,
c.chinese_stu as ch_stu,
main.course_id as course_id,
(i.indian_stu + j.japan_stu + c.chinese_stu) as total
from indian_student i, japan_student j,chinese_student c, course c
where i.course_id=main.course_id and j.course_id=main.course_id and c.course_id=main.course_id group by course_id;
can I get any advice please

Answer 1

You're using the 1990s-vintage method of joining tables together: comma-separated lists of tables. This is inadequate to your task. You need LEFT JOIN because the comma syntax is an INNER JOIN syntax. INNER JOIN omits rows without matches.
(Also, your sample query uses the c alias twice; that won't work. It also uses GROUP BY but I believe you want ORDER BY)
You want this:
select i.indian_stu as in_stu,
j.japan_stu as ja_stu,
x.chinese_stu as ch_stu,
main.course_id as course_id,
(i.indian_stu + j.japan_stu + c.chinese_stu) as total
from course c
left join indian_student i ON c.course_id = i.course_id
left join japan_student j ON c.course_id = j.course_id
left join chinese_student x on c.course_id = x.course_id
order by c.course_id
When you develop a view, first get the SELECT statement to work, then use it to create the view. That's easier than continually dropping and recreating the view.