How to use compound foreign keys with Postgresql

I am working on a multiple-choice quiz program. We have four tables related to questions and answers:

  • a table of questions
  • a table of answers
  • a table that links each question to multiple answers
  • a table that records a user’s actual answer to a question.

I needed a way to track answers to multiple-choice questions, and to limit possible answers to each question. For example, I want to make sure that if I ask the question “what is your favorite color?” then the answer is a color, and not something silly like “yes.” This is tricky, because “yes” is a valid answer for some questions, but not for others.

This table holds each question.

create table questions (
q_id serial primary key,
q_text text

Here are some questions. Since I used the serial data type, I’m not going to assign the q_id column. Postgresql will assign the q_id automatically.

insert into questions (q_text) values ('What is your favorite color?'); /* q_id 1 */
insert into questions (q_text) values ('Is it raining outside?'); /* q_id 2 */

After inserting, this is what the database looks like now:

select * from questions;
q_id | q_text
1 | What is your favorite color?
2 | Is it raining outside?
(2 rows)

This table lists every possible answer, each with a unique id.

create table all_answers (
a_id serial primary key,
a_text text

These are some answers.

insert into all_answers (a_text) values ('red'); /* a_id 1 */
insert into all_answers (a_text) values ('yes'); /* a_id 2 */
insert into all_answers (a_text) values ('green'); /* a_id 3 */
insert into all_answers (a_text) values ('no'); /* a_id 4 */

Here’s what all_answers looks like after adding data:

select * from all_answers;
a_id | a_text
1 | red
2 | yes
3 | green
4 | no
(4 rows)

This table links each question to meaningful possible answers. I am using question ID (q_id) and answer ID (a_id) numbers in order to save space.

create table possible_answers (
q_id integer references questions (q_id),
a_id integer references all_answers (a_id),
primary key (q_id, a_id)

Now, I’ll link certain questions with certain answers. This insert links ‘What is your favorite color?’ with ‘red’.

insert into possible_answers (q_id, a_id) values (1, 1);

This statement links ‘Is it raining outside?’ with ‘no’.

insert into possible_answers (q_id, a_id) values (1, 3);

And, just to continue the trend…

insert into possible_answers (q_id, a_id) values (2, 2);
insert into possible_answers (q_id, a_id) values (2, 4);

Now let’s see what we’ve got:

select * from possible_answers;
q_id | a_id
1 | 1
1 | 3
2 | 2
2 | 4
(4 rows)

Finally, this is the table that will record the actual answer given for a question and make sure that the answer is appropriate for the question.

create table real_answers (
q_id integer references questions (q_id), /* simple FK */
a_id integer references all_answers (a_id), /* simple FK */
foreign key (q_id, a_id) references possible_answers (q_id, a_id) /* compound FK */

Now watch what happens when I try to insert an answer that doesn’t match the question into the database. I’m going to try to answer the question “What is your favorite color?” with the answer “yes”.

Hopefully the database will prevent me.

select q_text from questions where q_id = 1;
What is your favorite color?
(1 row)

select a_text from all_answers where a_id = 2;
(1 row)

insert into real_answers (q_id, a_id) values (1, 2);
ERROR: $3 referential integrity violation - key referenced from real_answers
not found in possible_answers
Hurray! We are prevented from entering the data! Now, let’s try storing an acceptable answer.

insert into real_answers (q_id, a_id) values (1,1);
INSERT 17196 1

That means it worked!

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My name is Matt Wilson and I live in Cleveland Heights, Ohio. I love random emails from strangers, so get in touch!

  • These equations are very hard to solve but thanks to your posts and detail information its not that hard now:)

    Thanks you

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