如何将多个表链接到一个查询中

Question

I am in need of help. I never worked with complex databases, and I don't know how to arrange the tables. I need a table with children names, which is linked to their parents account (did this using $_POST['children_name'] , etc when submitting the registration form), but now I need to link their schools with their grades (example I am in 8 A grade), then their classes with the individual grade school subjects, and then link marks (like A+ in America) and absences to individual children.

I have no idea on how to do this. I made an example, which is illustrated in the photo below. Could you please help me?

(NOTE THAT THERE ARE 3 TABLES THERE)

Image

LATER EDIT: Fluffeh`s solution, a little bit edited! " I would consider the following structure for your tables.

First, a table for all the schools that the website will have data from.

id    schoolname
 1     European College#1
 2     European Colelge#2

Then, a table of all the classes that are active in a college.

id    Class
 1    8A
 2    8B
 3    8C

then a table for the subjects offered by all the schools. You will need an ID and then any other information as needed.

    schoolSubject
id  name
1   mathematics
2   geography
    3       physics
    4       geometry

Next up a table containing relations between the schools, classes and school subjects.

relations
SchoolID   ClassID     SubjectID
1           1               1
1           1               2
1           1               4

(so, if my logic is correct, class 8A students from the "European College#1" will study mathematics, geography and geometry)

Lastly, we will need to know which student is from where.

students
id  name        class_id   schoolname_id
1   John Doe     1           1
2   Jane Doe     2           2

The last tabe pretty much gives us the ability to have a record of multiple grades for the same student for the same subject - in case a student has to take a subject more than once.

grades
subjectID   studentID   grade
1           1           B,A,B
1           2           A,B,B
2           1           A,C,C
2           2           B,A,A

With this structure in place, what query could I use?

Answer 1

I would consider the following structure for your tables.

Firstly, a table for the subjects offered by the school. You will need an ID and then any other information as needed.

schoolSubject
id  name
1   mathematics
2   geography

Next up a table containing student information. Again, we have a unique ID, then any info as needed

student
id  name
1   John Doe
2   Jane Doe

The next table allows what is known as a many to many relationship. basically it means you can have multiple entries for the same subject (as many students can be enrolled in each) and multiple entries for students (as students can be enrolled in multiple subjects).

studentEnrolment
subjectID   studentID
1           1
1           2
2           1

The last tabe pretty much gives us the ability to have a record of multiple grades for the same student for the same subject - in case a student has to take a subject more than once.

grades
subjectID   studentID   grade
1           1           B
1           2           A
2           1           A
2           2           B

With this stucture in place, you can do a query like the following:

select
    subj.name,
    stud.name,
    grad.grade
from
    schoolSubject subj
        join studentEnrolment enr
            on subj.id=enr.subjectID
        join student stud
            on enr.studentID=stud.studentID
        join grades grad
            on subj.subjectID=grad.subjectID
            and stud.id=grad.studentID