我应该为抽认卡应用程序使用单独的表格吗？

Question

I'm designing a MySQL database for a flashcards app. I have a table called "courses", where i keep courses' ids, names and descriptions and a table called items, where I keep all the flashcards. The table items keeps a unique id, id of the course, as well as, questions, answers and more details. There is also a table called repetitions, where I store data which flashcards a user has memorised. If I want to get a flashcard a user hasn't memorised yet, I query the database with LEFT JOIN so I get all data from flashcards and rows from repetitions that match. However, I have to specify what course a certain flashcard belongs to, so my query looks more o less like that:

SELECT * 
    FROM (SELECT * 
              FROM flashcards 
              WHERE course='coursename') AS flashcards 
        LEFT JOIN repetitions 
            ON flashcards.id=repetitions.flashcardid 
    WHERE repetitions.lastrepetition IS NULL 
    LIMIT 1;

Would it be better to have separare tables for each course so I wouldn't have to specify what course I mean? Would that speed up the database? There won't be many courses (about 10-20) and each course would consist of about 5k-10k flashcards.

Sorry, I forgot to say that, if a user hasn't memorise a flashcard, there won't be any equivalent record in the table repetitions. What I actually do is: 1. I choose all flashards WHERE coursename = 'coursename', 2. I choose all repetitions WHERE user = 'user' AND coursename = 'coursname, 3. and then I LEFT JOIN the records from the first table with the records from the second table. I get all flashcards from a certain course and some of these records have their equivalents in the repetitions table, 4. then I use WHERE repetitions IS NULL and I get only those flashcards that don't have their uqivalents in the repetitions table.

    SELECT * FROM
    (SELECT * FROM flashcards WHERE course='coursename') AS f
    LEFT JOIN
    (SELECT * FROM repetitions WHERE user='user' AND course='coursename') AS r
    ON f.id=r.flashcardid
    WHERE repetitions IS NULL LIMIT 1)

The above query works fine, but I wonder whether it won't require too much RAM/memory, if there are more people using the app.

The only better solution I have atm is to choose only ids from the flashcards table and set the LIMIT to the user's limit (let's say 30 flashcards per session). Then instead of 30 times I'll use this query only once. And I can pass the id's in a GET variable. (The only problem with this solution is, that if a user studies flashcards on his PC and then moves to a mobile and forgets to switch off the PC's session and comes back to the PC, some repetitions might get overwritten. Anyway, this would be a very rare case, but still possible.)

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Thanks for your help! Your suggestions actually happend to be right. I've just figured out where I made a mistake - in my original query, I unnecessarily added 'WHERE repetitions.course='coursename' condition. However, I still have to use brackets for the right table to determine the user.

SELECT * FROM flashcards AS f
LEFT JOIN (SELECT * FROM repetitions WHERE user='username') AS r
ON f.id=r.flashcardid
WHERE f.course='coursename' AND r IS NULL

Answer 1

You should not split up the courses into there own tables.

couldn't you do something like this instead?

select * from flashcards
left join repititions
  on flashcards.id = repititions.flashcardid
where course = 'coursename' 
and repititions.lastrepitition is null
limit 1

Answer 2

I see no reason to add the overhead and headaches of a separate table for each course. Also, there's no need for that subquery. Your query can be written as:

SELECT *
    FROM flashcards f
        LEFT JOIN repetitions r
            ON f.id = r.flashcardid
    WHERE f.course = 'coursename'
        AND r.lastrepetition IS NULL
    LIMIT 1;