How do I fix foreign key constraints - sql

Question

I am creating a multiple choice quiz database and when I am trying to create CorrectAnswer table I am getting the following error:
Msg 1785, Level 16, State 0, Line 15
Introducing FOREIGN KEY constraint 'FK__CorrectAn__Answe__5BE2A6F2' on table 'CorrectAnswer' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.
Msg 1750, Level 16, State 1, Line 15
Could not create constraint or index. See previous errors.
CREATE DATABASE PeriodicTableQuiz
CREATE TABLE Question(
QuestionID INT IDENTITY(1,1) PRIMARY KEY,
QuizQuestion VARCHAR(MAX) DEFAULT NULL
);
CREATE TABLE AnswerChoices(
AnswerID INT IDENTITY(1,1) PRIMARY KEY,
Answer VARCHAR(MAX) DEFAULT NULL,
QuestionID INT NOT NULL,
FOREIGN KEY(QuestionID) REFERENCES Question(QuestionID) ON DELETE CASCADE ON UPDATE CASCADE
);
CREATE TABLE CorrectAnswer(
CorrectAnswerID INT IDENTITY(1,1) PRIMARY KEY,
QuestionID INT NOT NULL,
FOREIGN KEY(QuestionID) REFERENCES Question(QuestionID) ON DELETE CASCADE ON UPDATE CASCADE,
AnswerID INT NOT NULL,
FOREIGN KEY(AnswerID) REFERENCES AnswerChoices(AnswerID) ON DELETE CASCADE ON UPDATE CASCADE
);
I want to insert questions in the Question table and answers in the AnswersChoices table. CorrectAnswer should have QuestionID and AnswerID referencing Question and AnswerChoices tables, respectively.

Answer 1

While it's true that you simply don't need a QuestionID column in CorrectAnswer since it is redundant, and the following design would solve the current problem:
CREATE TABLE dbo.CorrectAnswer(
CorrectAnswerID INT IDENTITY(1,1) PRIMARY KEY,
AnswerID INT NOT NULL,
FOREIGN KEY(AnswerID) REFERENCES dbo.AnswerChoices(AnswerID)
ON DELETE CASCADE ON UPDATE CASCADE
);
I propose this design instead:
CREATE TABLE dbo.Questions
(
QuestionID INT IDENTITY(1,1) PRIMARY KEY,
Question VARCHAR(MAX) DEFAULT NULL
);
CREATE TABLE dbo.Answers
(
AnswerID INT IDENTITY(1,1) PRIMARY KEY,
Answer VARCHAR(MAX) DEFAULT NULL,
QuestionID INT NOT NULL,
IsCorrectAnswer bit NOT NULL,
CONSTRAINT FK_Question FOREIGN KEY(QuestionID)
REFERENCES dbo.Questions(QuestionID)
ON DELETE CASCADE ON UPDATE CASCADE
);
A constraint can't enforce that only one row can be correct per question, but you can enforce this through a trigger (scope creep for this question). Enforcing at least one row can be a little trickier (much like enforcing that a row must exist in CorrectAnswer in your original design, because when can you do that?), simply because it would mean you have to insert the correct answer first.
You're probably also in need of some metadata to define the order choices appear in the quiz, especially if you do intend to insert correct / incorrect answers in some predictable order.

Answer 2

Because Question is required, all one-to-many relationships where Question is involved is having cascading delete enabled. It means, if you delete a Question
the delete will cascade directly to AnswerChoices
the delete will cascade directly to CorrectAnswer and because CorrectAnswer and
AnswerChoices have a required one-to-many relationship with cascading
delete enabled it will then cascade from AnswerChoices to
CorrectAnswer
So, you have two cascading delete paths from Question to CorrectAnswer - which causes the exception.
You must either make the QuestionId optional in at least one of the tables.
I propose the following:
CREATE DATABASE PeriodicTableQuiz
CREATE TABLE Question(
QuestionID INT IDENTITY(1,1) PRIMARY KEY,
QuizQuestion VARCHAR(MAX) DEFAULT NULL
);
CREATE TABLE Answer(
AnswerID INT IDENTITY(1,1) PRIMARY KEY,
Answer VARCHAR(MAX) DEFAULT NULL,
);
CREATE TABLE CorrectAnswer(
CorrectAnswerID INT IDENTITY(1,1) PRIMARY KEY,
QuestionID INT NOT NULL,
FOREIGN KEY(QuestionID) REFERENCES Question(QuestionID) ON DELETE CASCADE ON UPDATE CASCADE,
AnswerID INT NOT NULL,
FOREIGN KEY(AnswerID) REFERENCES AnswerChoices(AnswerID) ON DELETE CASCADE ON UPDATE CASCADE
);
CorrectAnswer is now having one to many relationship. Deleting a question will delete the correctanswer rows. An answer can now be associated with different questions.
Again, it depends on your requirements whether an answer is a child of question or is independent.