Question

Question

Is the set up for foreign keys I want to do for Answer Table the correct Idea and how to write SQL on Add foreign key constraint dealing with multiple fields?

Session Table:

SessionId (PK) SessionName
1              AAA
2              AAB

Question Table:

QuestionId(PK)  SessionId(PK)  QuestionContent
1               1              What is 2+2?
2               1              What is 3+3?
1               2              What is 4+4?
2               2              What is 5+5?

Answer Table:

AnswerId(auto, PK)  QuestionId(fk)  SessionId(fk)  Answer
1                   1               1              A
2                   1               1              D
3                   2               1              C
4                   2               1              A
5                   1               2              True
6                   2               2              A
7                   2               2              B

Now the issue I have is with foreign keys with the Answer Table, I want to know which foreign key is correct to do.

What I have done is that I add a foreign key to the SessionId which means that if I click on a row’s SessionId, it matches the SessionId in the Session Table.

Then I thought I will provide a foreign key on QuestionId in Answer Table linking to Question Table. Problem with this though is that if within a row I click on a QuestionId, then it will display me all of the rows in the QuestionId which contains the same QuestionId. So if I wanted to look up this Answer:

AnswerId(auto, PK)  QuestionId(fk)  SessionId(fk)  Answer
1                   1               1              A

When I click on QuestionId Foreign key above it outputs this below:

QuestionId(PK)  SessionId(PK)  QuestionContent
1               1              What is 2+2?
1               2              What is 4+4?

The Answer does not belong to both of these questions, it only belongs to one question but because both QuestionId are the same, it displays both questions.

So what I am thinking is that I want a foreign key constraint where that if user clicks on the QuestionId in a row, it will look up both QuestionId and SessionId so that it knows that question and session (exam) that answer belongs to so that it outputs below:

QuestionId(PK)  SessionId(PK)  QuestionContent
1               1              What is 2+2?

My question is that I don’t know how to write a foreign key constraint which deals with two fields? Also is the set up of foreign keys for just SessionId and SessionId and QuestionId together correct?

Answer 1

You can make a composite foreign key for your Answers table using this syntax:

FOREIGN KEY (QuestionId, SessionId) REFERENCES Questions (QuestionId, SessionId)

However, it seems strange that your SessionId column for the Questions table is part of a composite Primary Key, considering you have a Sessions table using a column by that name as the Primary Key. You should probably consider making the Questions.SessionId a foreign key on your Sessions table, and have QuestionId be a standard single Primary Key.

If you do this, you’ll find that you won’t need a SessionId column on your Answers table.

Here’s an example of what you might do:

-- Sessions table
SessionId (PK) | SessionName

-- Questions table
QuestionId (PK) | SessionId (FK) | QuestionContent

-- Answers table
AnswerId (PK) | QuestionId (FK) | Answer