Question

I have an SQL table that links students to classes. This table primarily has the ids user_id and class_id. Let’s suppose that each user_id is unique to the table but class_ids are not.

I want to retrieve all the instances of user_id and group them by class_id. I also wanted to be able to reprensent the result returned by SQL as the following in PHP:

array(
class_id => array(
    user_id => <user info..>,
    user_id => <user info..>,
    user_id => <user info..>
),
class_id => array(
    user_id => <user info..>,
    user_id => <user info..>,
    user_id => <user info..>
),
class_id => array(
    user_id => <user info..>,
    user_id => <user info..>,
    user_id => <user info..>
)

)

I know I could call an SQL inject per class and return the users from that class and format the result into the format I need in PHP, but would there be a way to do this in one SQL request as oposed to having to do one request per class (+ 1 request to get the list of class ids)?

It is because I need to access specific classes, so I think that the key/value solution of arrays in PHP are the best way to access classes.

Answer 1

The short answer is no. If you use a LEFT JOIN, you could get one result set with both class info and student info however. it would not be in the exact format above, but would be a pretty simple conversion.

the result set would look like

class_id, student_id, student_etc
class_id, student_id, student_etc
class_id, student_id, student_etc

there would be one record for each class * number of students taking that class

to elaborate

classes table
id, etc

users table
id, etc

class_to_users table
id, class_id, user_id

then

SELECT classes.id, users.id
FROM classes
LEFT JOIN class_to_users ON class_to_users.class_id = classes.id
LEFT JOIN users ON class_to_users.user_id = users.id
WHERE 1
ORDER BY classes.id