MySQL data structure for questions needing multiple inputs to be true - mysql

Question

Apologies for the title as I'm a bit unsure how to phrase this myself so hopefully an example might help.
I've got a MySQL table that holds questions
question_id, PK Int
text - Text
I also have a table called value that looks like this
value_id, PK Int
value - varchar
I think I might need a mapping table for this along the lines of
question_to_value
question_id int
value_id int
Though if my example looks like I don't need one then I can change the structure
Basically, given a single or multiple value_ids I want to pick the question that should be asked
so if I am given value_ids 1,2 I should have a unique question_id relating to those ids in the database. Given a value_id of 3 there should be a different question_id, and an input of value_ids (1,2,3) or 1,3 should again retrieve unique question_ids for both permutations.
I'm struggling with how I should go about it. Should I use a) a sort of joining table for this, and b) Most efficient way of querying it.
My initial thought it to have a question_to_value table that holds a question_id and value_id on a 1-1 basis, then doing the following
select question_id from question WHERE value_id in (?,?,?) but i'm not sure if this is the optimal way to structure this. Especially as the trouble is using the 'IN' query above if I'm just given the value_id of 1, it would actually bring back all the questions where value_id '1' is the only or part of the group of values to product a particular question. e.g
question_id 1 maps to value 1
question_id 2 maps to values of 1 and 2.
my in statement would bring back question_ids 1 and 2 for a value_id of 1 when I only want question_id 1 as it should match all criteria.
Any ideas on how I should structure this?
*** editing I'm trying to come up with another way of phrasing this to avoid confusion so hopefully the following will help
consider I have 4 'questions'
a
b
c
d
If I'm given an input of 1 I only want to retrieve a.
If I'm given 2 inputs of 1 and 2 I only want to retrieve b.
If I'm given 3 inputs of 1,2,3 I only want to retrieve c.
If I'm given 2 inputs of 1 and 3 I only want to retrieve d.

Answer 1

I'm trying to break the problem apart into component parts to define the question in a way that can be expressed in code.
I'm working on the idea that the proper question must match two rules:
The question must have an entry for each of the input values
The question must not have entries for any other input value
First, consider a query that returns all the value ids for the inputs.
SELECT value_id from values where `value` IN <inputs>
I don't know what language you are using, so I can't tell you how to build the inputs list. In php, it would be something like:
"('" . implode("','" $inputs) . "')"
to properly wrap each option in quotes (assuming the values don't also have quotes, but that's a separate, language-specific problem to solve).
Now it is simple to use that query to create a query that returns questions that have ANY of the input values:
SELECT question_id
FROM mapping_table
WHERE value_id IN (SELECT value_id from values where `value` IN <inputs>)
Finally, we can tweak that query to only return the question_ids with the right number of matches. We want one row for each question, and we only want questions that match all the values:
SELECT question_id
FROM mapping_table
WHERE value_id IN (SELECT value_id from values where `value` IN <inputs>)
GROUP BY question_id
HAVING COUNT(question_id) = <number of inputs>
That will give you a list of question_ids that match constraint 1 above. It will not test for constraint 2 in every case; if the input is (1,2) then this will match the question for (1,2) and the question for (1,2,3).
However, if you only get one row back from this query, there is no need to filter that list with the second query.
If you did get more than one match for the above, the second query is pretty simple - it's similar to the above query but with a different WHERE clause. This will get all the mapping table entries for the ids from the first query, and match the ones that don't have any extras:
SELECT question_id
FROM mapping_table
WHERE question_id IN <ids from first query>
GROUP BY question_id
HAVING COUNT(question_id) = <number of inputs>
You could combine the two queries, but unless performance is an issue, I'd keep them separate to make things easier to maintain.
NOTE: there is nothing here to constrain your data to make sure it is valid. In other words, there's nothing to prevent two questions from matching (1,2) when the questions are added to the database.