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.