问题
I have one database with users and one with questions. What I want is to ensure that every user can answer every question only once.
I thought of a database that has all the question id's as columns and all the user id's as records, but this gets very big (and slow I guess) when the questions and the user count grow.
Is there another way to do this with better performance?
回答1:
You probably want a setup like this.
Questions table (QuestionID Primary Key, QuestionText)
Users table (UserID Primary Key, Username)
Answers table (QuestionID, UserID, Date) -- plus AnswerText/Score/Etc as needed.
In the Answers table the two first columns together form a compound primary key (QuestionID, UserID) and both are foreign keys to Question(QuestionID) and Users(UserID) respectively.
The compound primary key ensures that each combination of QuestionID/UserID is only allowed once. If you want to allow users to answer the same question multiple times you could extend the ¨compound primary key to include the date (it would then be a composite key).
This is a normalized design and should be efficient enough. It's common to use a surrogate primary key (like AnswerID) instead of the compound key and use a unique constraint instead to ensure uniqueness - the use of a surrogate key is often motivated by ease of use, but it's by no means necessary.
回答2:
Diagram
Below is a diagram of my own table design, quite similar to the correct Answer by jpw. I made up a few column names to give more of a flavor of the nature of the table. I used Postgres data types.
As the last paragraph of that Answer discusses, I would go with a simple single primary key on the response_ ("Answers") table rather than a compound primary key combining fkey_user_ & fkey_question_.
Unrealistic
This diagram fits the problem description in the Question. However this design is not practicable. This scenario is for a single set of questions to be put to the user, only a single survey or quiz ever. In real life in a situation like a school, opinion survey, or focus group, I expect we would put more than one questionnaire to a user. But I will ignore that to directly address the Question as worded.
Also in some scenarios we might have versions of a question, as it is tweaked and revised over time when given on successive quizzes/questionnaires.
Performance
Your Question correctly identifies this problem as a Many-To-Many relationship between a user and a question, where each user can answer many questions and each question may be answered by many users. In relational database design there is only one proper way to represent a many-to-many. That way is to add a third child table, sometimes called a "bridge table", with a foreign key linking to each of the two parent tables.
In a diagram where you draw parent tables vertically higher up the page than child tables, I personally see such a many-to-many diagram as a butterfly or bird pattern where the child bridge table is the body/thorax and the two parents are wings.
Performance is irrelevant in a sense, as this is the only correct design. Fortunately, modern relational databases are optimized for such situations. You should see good performance for many millions of records. Especially if you a sequential number as your primary key values. I tend to use UUID data type instead; their arbitrary bit values may have less efficient index performance when table size reaches the millions (but I don't know the details.
来源：https://stackoverflow.com/questions/31620869/database-design-for-many-to-many-relations-with-restrictions