New to databases, how do I design this? MYSQL and Java

Question

I am new to databases and before I start learning mySQL and using a driver in Java to connect with my database on the server-side, I wanted to get the design of my database down first. I have two columns in the database, CRN NUMBER, and DEVICE_TOKEN. The CRN number will be a string of five digits, and the DEVICE_TOKEN will be a string device token(iOS device token for push notifications). Let me try to describe what I am trying to do. I am going to have users send my server data from the iOS app, mainly their device token for push notifications and a CRN(course) they want to watch. There are going to be MANY device tokens requesting to watch the same CRN number. I wanted to know the most efficient way to store these in a database. I am going to have one thread looking through all of the rows in the DB, and polling the website for each CRN. If the event I am looking for takes place, I want to notify every device token associated with this CRN. Initially, I wanted to have one column being the CRN, and the other column being DEVICE_TOKENS. I have learned though that this is not possible, and that each column should only correspond to one entry. Can someone help me figure out the best way to design this database, that would be the most efficient?

CRN     DEVICE_TOKEN

12345   "string_of_correct_size"
12345   "another_device_token"

Instead of me making multiple request to the website for the same CRN, it would be MUCH more efficient for me to poll the website per unique CRN ONCE per iteration, and then notify all device tokens of the change. How should I store this information? Thanks for your time

Answer 1

The most normal way to do this would be to normalize out the Courses with a foreign key from the device tokens. Eg Two tables:

Courses
id    CRN
1     12345

InterestedDevices
id    course    DEVICE_TOKEN
1     1        "string_of_correct_size"
2     1        "another_device_token"

You can then find interested devices with a SQL like the following:

SELECT *
FROM Courses
JOIN InterestedDevices ON Courses.id = InterestedDevices.course
WHERE Courses.CRN = ?

This way you avoid duplicating the course information over and over.

Answer 2

In this type of problem where you have a one-to-many relationship (one CRN with many Device_Tokens), you want to have a separate table to store the CRN where a unique ID is assigned for each new CRN. A separate table should then be made for your DEVICE_TOKENS that relates has columns for a unique ID, CRN, and DEVICE_TOKEN.

With this schema, you can go through the rows of the CRN table, poll against each CRN, and then just do a simple JOIN with the DEVICE_TOKEN table to find all subscribed devices if a change occurs.