Intersection of Records in Postgres - sql

Question

Suppose I have labels with multiple stores associated with them like so:
label_id | store_id
--------------------
label_1 | store_1
label_1 | store_2
label_1 | store_3
label_2 | store_2
label_2 | store_3
label_3 | store_1
label_3 | store_2
Is there any good way in SQL (or jooq) to get all the store ids in the intersection of the labels? Meaning just return store_2 in the example above because store_2 is associated with label_1, label_2, and label_3? I would like a general method to handle the case where I have n labels.

Answer 1

This is a relational division problem, where you want the stores that have all possible labels. Here is an approach using aggregation:
select store_id
from mytable
group by store_id
having count(*) = (select count(distinct label_id) from mytable)
Note that this assumes no duplicate (store_id, label_id) tuples. Otherwise, you need to change the having clause to:
having count(distinct label_id) = (select count(distinct label_id) from mytable)

Answer 2

Since you're also looking for a jOOQ solution, jOOQ supports a synthetic relational division operator, which produces a more academic approach to relational division, using relational algebra operators only:
// Using jOOQ
T t1 = T.as("t1");
T t2 = T.as("t2");
ctx.select()
.from(t1.divideBy(t2).on(t1.LABEL_ID.eq(t2.LABEL_ID)).returning(t1.STORE_ID).as("t"))
.fetch();
This produces something like the following query:
select t.store_id
from (
select distinct dividend.store_id
from t dividend
where not exists (
select 1
from t t2
where not exists (
select 1
from t t1
where dividend.store_id = t1.store_id
and t1.label_id = t2.label_id
)
)
) t
In plain English:
Get me all the stores (dividend), for which there exists no label (t2) for which that store (dividend) has no entry (t1)
Or in other words
If there was a label (t2) that a store (dividend) does not have (t1), then that store (dividend) would not have all the available labels.
This isn't necessarily more readable or faster than GROUP BY / HAVING COUNT(*) based implementations of relational divisions (as seen in other answers), in fact, the GROUP BY / HAVING based solutions are probably preferrable here, especially since only one table is involved. A future version of jOOQ might use the GROUP BY / HAVING approach, instead: #10450
But in jOOQ, it might be quite convenient to write this way, and you asked for a jOOQ solution :)

Answer 3

Then convert the query by #GMB into an SQL function that takes an array and returns a table of store_id's.
create or replace
function stores_with_all_labels( label_list text[] )
returns table (store_id text)
language sql
as $$
select store_id
from label_store
where label_id = any (label_list)
group by store_id
having count(*) = array_length(label_list,1);
$$;
Then all that's needed is a simple select. See complete example here.

Answer 4

If there are three particular labels you want, you can use:
select store_id
from t
where label in (1, 2, 3)
group by store_id
having count(*) = 3;
If you want only those three labels and nothing else, then:
select store_id
from t
group by store_id
having count(*) = 3 and
count(*) filter (where label in (1, 2, 3)) = count(*);