Question

Assume you have a relation that needs to somehow reference any (variable) number of different items in another table (by their keys). How would this be achieved?

Answer 1

For this purpose you would usually have an intermediate table. Here is an example schema

Table - Question
QuestionID
Title
Body
Date

Table - Tag
TagID
TagName

Table - QuestionTag
QuestionID
TagID

In a schema such as this, The question table contains questions. The tag table contains a list of tags that can be applied to the question (like on this site). And the QuestionTag links the two together, so that a question can be linked to any number of tags. To get a specific questions with it’s tags, you could do the following

SELECT Question.QuestionID, Question.Title,Question.Body,Question.Date,GROUP_CONCAT(Tag.TagName)
FROM Question
LEFT JOIN QuestionTag 
ON Question.QuestionID = QuestionTag.QuestionID
LEFT JOIN Tag 
ON QuestionTag.TagID = Tag.TagID
WHERE Question.QuestionID=@QuestionID
GROUP BY Question.QuestionID