How to maintain referential integrity with common great-parent table?

Question

Lets say I have a table with "Groups of Questions"

GroupID | value
--------+------
42      | How often do you
9071    | Other question
...

And, for each group of questions I have "questions" and "possible answers"

Group | QuestionID | value
------+------------+------
42    | 5          | ... brush your teeth?
42    | 89         | ... go to the movies?
9071  | 709        | ... another question ...
...

Group | Answer | value
------+--------+------
42    | 134    | several times per day
42    | 135    | twice a day
42    | 71     | once a day
42    | 803    | every other day
42    | 8      | once a week
42    | 666    | once a month
...

Now, in PHP/HTML, I do a (virtual) full cross product of questions and possible answers of group 42 to build a 2-entry table where the user will select his/her answers (HTML version)

How often do you      | N/d | 2/d | 1/d | d*2 | 1/w | 1/m |
----------------------+-----+-----+-----+-----+-----+-----+
... brush your teeth? | ( ) | (X) | ( ) | ( ) | ( ) | ( ) |
... go to the movies? | ( ) | ( ) | ( ) | ( ) | (X) | ( ) |

I know I'll need to
insert into answers (question, answer, ...) values (5, 135, ...) for teeth
and
insert into answers (question, answer, ...) values (89, 8, ...) for movies

The question is: is there a way to enforce that both question and answer, in the answers table, "belong" to the same group of questions?

I'd like that
insert into answers (question, answer, ...) values (709, 71, ...)
be disallowed, because question 709 "belongs" to group 9071 and answer 71 belongs to group 42.

Answer 1

Well, the answer relates to the question, not to the group (at least not directly), so I would have thought the "possible answers" table should be:

Question | Answer | value
---------+--------+------
5        | 134    | several times per day

Then when you insert an answer, you give the QuestionId and AnswerId, and together they reference the "possible answer" and thereby, indirectly, the group.

Answer 2

You include the columns Group, QuestionID and Answer then create two foreign keys, one on (Group, QuestionID) and the other on (Group, Answer).