Find unique sets in sql data - mysql

Question

I have three tables:
Modules
| ID | name |
Subscription
| module_id | user_id | ...
User
| ID | user_name |
I need a list of unique subscription sets. (ie. x users subscribed to modules (1), y users subscribed to (1,2), etc. Can I do this in SQL?

Answer 1

Let's set up some tables.
create table modules (
module_id integer not null,
module_name varchar(15) not null,
primary key (module_id),
unique (module_name)
);
insert into modules values (1, 'First module');
insert into modules values (2, 'Second module');
create table users (
user_id integer not null,
user_name varchar(15) not null,
primary key (user_id),
unique (user_name)
);
insert into users values (100, 'First user');
insert into users values (101, 'Second user');
create table subscriptions (
module_id integer not null,
user_id integer not null,
primary key (module_id, user_id),
foreign key (module_id)
references modules (module_id),
foreign key (user_id)
references users (user_id)
);
insert into subscriptions values (1, 100);
insert into subscriptions values (1, 101);
insert into subscriptions values (2, 100);
To get the right counts, all you need is a query on the table "subscriptions".
select module_id, count(*) as num_users
from subscriptions
group by module_id
order by module_id;
module_id num_users
--
1 2
2 1
You can use that statement in a join to get the module name instead of the module id number.
select t1.module_name, t2.num_users
from modules t1
inner join (select module_id, count(*) as num_users
from subscriptions
group by module_id
) t2
on t1.module_id = t2.module_id
order by t1.module_name;
module_name num_users
--
First module 2
Second module 1
You'll need to use the Innodb engine in order to enforce foreign key constraints.
To get the users who are subscribed to both module ids 1 and 2, use a WHERE clause to pick the module id numbers, use a GROUP BY clause to get the count, and use a HAVING clause to restrict the output to only those user id numbers who have a count of 2. (That means they've subscribed to both those modules in the WHERE clause.)
select user_id, count(*) num_modules
from subscriptions
where module_id in (1, 2)
group by user_id
having count(*) = 2;
This kind of requirement can blow up in your face quite quickly if you need to report on all the possible combinations of modules. For only 10 modules, there are over 1000 possible combinations. Usually, you'd want to write a program to either
generate dynamic SQL,
generate a static SQL statement for each of the possible combinations,
write a new SQL statement each time you're asked to report on a combination (usually, most combinations aren't interesting), or
renegotiate the requirements.

Answer 2

My colleague came up with an interesting solution
select module_combinations
from (select user_id, group_concat(module_id separator ', ') as module_combinations from subscriptions group by user_id) a
group by a.module_combinations
But this seems closer to answering the original question.
select module_combinations, count(*) as num_users
from (select group_concat(module_id order by module_id) as module_combinations
from subscriptions
group by user_id) a
group by a.module_combinations;