CODEWITHSUNDEEP
sqlmysql
medium
medium

Reputation: 4236

MYSQL query INNER JOIN two table

I have three tables CLASSES, CHILD_CLASSES, and STUDENTS.

CLASSES looks like this (its self referencing):

CLASS_ID | CLASS_PARENT |
-------------------------
    1    |      ---
    2    |       1 
    3    |       2

CHILD_CLASSES looks like this:

CC_ID | PARENT_CLASS_ID | CHILD_CLASS_ID
----------------------------------------
  1   |       1         |     2    
  2   |       1         |     3
  3   |       2         |     3

STUDENTS looks like this

 STU_ID | CLASS_ID | STU_NAME
 ----------------------------
    1   |     1    |     A
    2   |     1    |     B
    3   |     2    |     C

Basically the CLASSES table is self referencing but each CLASS can also have children associated with that class. So CLASS 1 has children 2 and 3 and Class 2 has children of just 3.

So a many students can have the same class id. The students are also allowed to see all information that is a child of the class_id that is associated with them so any student with class ID of 1 can see classes 1, 2 and 3 but a student with a class idea of 2 can only see classes with ID 2 and 3. I am trying to find a query to count the number of users of a particular class that have access. so if I was looking at class_id of 1 the query would return 3 because it has two children in the CHILD_CLASSES table and then itself in the classes table. I cant seem to get this to work though, I always come up short because due to trying to count the 1 user in the CLASSES table.

the query I am using so far is this

SELECT COUNT(class_id) as TOTAL 
  FROM students 
 WHERE class_id IN (SELECT class_id 
                      FROM child_classes 
                     WHERE parent_class_id = 1);

Upvotes: 1

Views: 307

Answers (1)

Peter Lang
Peter Lang

Reputation: 55524

Looks like child_classes contains the same data as classes, so the following should work:

SELECT COUNT(*)
FROM students
WHERE class_id = 1
   OR class_id IN ( SELECT child_class_id
                    FROM child_classes
                    WHERE parent_class_id = 1
                  )

It returns the count of all students which have class_id = 1 or are in a class which has parent_class_id = 1.

Upvotes: 1

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