MySQL to fetch related users

Question

I have two tables Schools and Users

Table School

-----------------------------------------------------
| id |     name    |  city  |     major    | userID |
----------------------------------------------------|
| 1  |  school A   | chicago |    CS       |    1   |
----------------------------------------------------|
| 2  |  school B   | chicago |    CS       |    1   |
----------------------------------------------------|
| 3  |  school A   | chicago |    CS       |    2   |
----------------------------------------------------|
| 4  |  school C   | chicago |    Art      |    2   |
----------------------------------------------------|
| 5  |  school B   | chicago |    CS       |    3   |
----------------------------------------------------|
| 6  |  school D   | chicago |    Math     |    3   |
----------------------------------------------------|
| 7  |  school A   |New York |    CS       |    3   |
----------------------------------------------------|
| 8  |  school B   | chicago |    Art      |    3   |
-----------------------------------------------------

Table Users

--------------------
| id |     name    |
____________________
| 1  |    User A   |
____________________
| 2  |    User B   |
____________________
| 3  |    User C   |
____________________
| 4  |    User D   |
____________________
| 5  |    User E   |
____________________

The userID field in the schools table is a foreign key to the id field in the users table. I want to write a MySQL statement that takes a given userID and lists all classmates of that user.

So, for the example above, a class mate of User A (ID#1) is a user that went to the same school as User A , is located in the same city , and has the same major . Thus, valid classmates for User A are only User B (ID#2) AND User C (ID#3).

Right now, I am using two MySQL statements to accomplish this goal. The first one is this

SELECT id FROM schools WHERE userID = '1'

which lists all schools for User A . Then I use PHP to loop through the results and for each row I run the following

    SELECT userID from schools WHERE 
name city LIKE '%$chicago%' 
AND name LIKE '%$school A%' 
AND major LIKE '%$CS%'

This works fine and returns the right list of userIDs. However, I am wondering if there is a more efficient way to do this in one SQL statement and without having to use PHP.

Answer 1

You can do it like this:

select distinct u.name
from Users u
join School s on s.userID = u.id
join (
            select distinct s.name, s.city
            from School s
            inner join Users u on u.id = s.userID
            where u.name = 'User A'
           ) aux on s.name = aux.name and s.city = aux.city
where u.name <> 'User A'

In the aux query you select the name 's and city 's of a @user and then select all users with those requirements and that aren't the @user itself.

You may replace 'User A' by a variable.

Answer 2

Your SCHOOL table is not considered right. It violates the rules of relational database management systems.

One must make 3 more tables: one for school name, another for city name, and a third for major name. All of these must be different tables with ids.

Clearly name column and city column have a many-to-many relationship so one must make a lookup table for this.

The same applies for major column. It seems like a lot of tables but then your extraction of any value will be simple.

OK, I made some changes to your tables. I prefer this way to store the data:

table student
col.
id 
name
major
reg.no.   (to make each student unique)

table school
col.
id 
name

table city
col.
id 
name

table student_school (lookup table)
col.
student_id
school_id

table school_city
col.
school_id
city_id

Prefer these tables and you will find that every query can be made with ease. If you have any problem with understanding this, let me know.