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Is using multiple foreign keys seperated by commas wrong, and if so, why?

I'm building online examination system. I have designed to table, Question and GeneralExam. The table GeneralExam contains info about the exam like name, description, duration,... Now I would like to design table GeneralQuestion, it will contains the id of questions belongs to a general exam.

Currently, I have two idea to design GeneralQuestion table:

  1. It will have two column: general_exam_id, question_id.
  2. It will have two column: general_exam_id, list_question_ids(string/text).

So, I would like to know which designing is better, or Pros and Cons of each designing. I'm using Postgresql database.

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Thanh
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    I wouldn't even really care about the performance - but if you use just a plain old list of strings - how do you ensure data integrity?? How can you make sure only valid ID's will be stored?? That's what foreign keys are for - ensuring data integrity (not performance....) – marc_s Nov 16 '12 at 11:23
  • @marc_s I have replied on SO, sorry for make question on many sites, because I think some people only care about some site :) – Thanh Nov 16 '12 at 11:28
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    See this question: Is using multiple foreign keys seperated by commas wrong, and if so, why? – ypercubeᵀᴹ Nov 16 '12 at 11:29
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    And at SO: Is storing a comma separated list in a database column really that bad? – ypercubeᵀᴹ Nov 16 '12 at 11:30
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    I have to repeat this as well: do *not* store comma separated values in a single column. Just don't. –  Nov 16 '12 at 11:38
  • @a_horse_with_no_name, thanks for advice, i designed table in first solution, but suddenly solution 2 coming in mind, so i just wonder maybe it better when it take less rows :) but now I know this is wrong, absolute wrong. – Thanh Nov 16 '12 at 11:44
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    Don't worry about 100, 1000 or even 10'000 rows - any decent RDBMS will just laugh at such a number of rows..... being able to enforce data integrity is *MUCH* more important than having a few lookup tables with a few hundred or a few thousand rows more..... having dealt with loads of crappily designed databases without data integrity, I cannot stress enough just how important data integrity (and thus referential integrity on a database level) is ..... – marc_s Nov 16 '12 at 12:55
  • @marc_s thanks so much :), i'm still be student, so maybe i had a stupid question, thanks to you helped me have more experience :D – Thanh Nov 16 '12 at 12:59
  • @Kien: No reason to apologize. It's better to ask questions when one isn't sure. And don't misunderstand if this question is closed. That shouldn't stop you from asking more of them :) – ypercubeᵀᴹ Nov 16 '12 at 14:44

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As ypercube and marc_s pointed out, it's best to have a 2-column "mapping" table to store which questions are used by which general exams.

This approach provides the following benefits:

  • Foreign Keys can then be used to ensure data integrity
  • Queries are much easier to write (eg: find all general exams that use question 17)
  • Extending the database schema at a later date (to support new features) will be much easier
  • The schema will be easier to understand by new/other developers/DBAs since it confirms to best practice
SteB
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