两列的唯一约束

Question

I have a table called QuestionMultipleChoice that has the following structure:

ID int,
QID int,
Text varchar(200),
CorrectAnswer bit

I'm wanting to establish a unique constraint that consists of QID and CorrectAnswer . However, I only want to limit where CorrectAnswer is '1'.

CONSTRAINT QuestionMultipleChoice_UC UNIQUE (QuestionID, CorrectAnswer)

Answer 1

If you set CorrectAnswer to null instead of 0, then a unique index on (QID,CorrectAnswer) will only consider rows where CorrectAnswer is 1.

From the SQL-92 standard

"A unique constraint is satisfied if and only if no two rows in a table have the same non-null values in the unique columns. In addition, if the unique constraint was defined with PRIMARY KEY, then it requires that none of the values in the specified column or columns be the null value."

Answer 2

You can do this using a Function Based Check Constraint or a Trigger .

Something like this (I might have missed on the syntax):

A function that returns the number of correct answers for a question:

CREATE FUNCTION CheckAnswer(pQuestionId int)
RETURNS int
AS 
BEGIN
   DECLARE @retval int
   SELECT @retval = COUNT(*) FROM QuestionMultipleChoice c where c.QuestionId = pQuestionId and c.CorrectAnswer = 1
   RETURN @retval
END;

A constraint that uses the function to check whether this question has only one correct answer. The check for CorrectAnswer = 0 is for a slight optimization:

ALTER TABLE QuestionMultipleChoice
ADD CONSTRAINT chkAnswer CHECK (CorrectAnswer = 0 OR dbo.CheckAnswer(QuestionId) <= 1);