Question

I’m trying to think of a solution to a mapping problem in which I can relate two seemingly unrelated groups. So, lets say I have a set of attributes, A, an Entity Group 1, and an Entity Group 2. I could easily have a many-many relationship between 1 and A and do queries like

select * from 1 
  inner join a_1 on a_1.1_id = 1.id
  inner join a on a.id = a_1.a_id
  where a.attr = '123'

I could obviously do something similar with A related to 2. What I’m trying to actually query though is A to 2 and A to 1 where the elements in group 1 and 2 share the same attribute in A. So, if the attribute I’m storing is an address, I want to be able to select out all people in group 1 and group 2 that share addresses, and what those addresses are.

Appreciate the help!

EDIT:

Table structure:

t1 -----< t1_A >------ A ------< t2_A >------ t2

t1[id, other], t1_A[id, t1id, Aid], A[id, address], t2_A[id, t2id, Aid], t2[id, other]

And the goal is to get a row returned for every element in t1 that has the same address as an element in t2(as well as what that address is).

Answer 1

query though is A to 2 and A to 1 where the elements in group 1 and 2
share the same attribute in A.

I think you are looking for UNION:

select address, ... -- and other properties
from table1 t1 
inner join a on a.a_id = t1.id
where a.attr = '123'
UNION ALL
select address, ... -- and other properties
from table2 t2 
inner join a on a.a_id = t2.id
where a.attr = '123'

---

Update:

Considering the tables’ structures, after updating your question:

t1:

• id,
• …

t1_A:

• id,
• t1id,
• Aid,
• …

A:

• id,
• address,
• …

t2_A:

• id,
• t2id,
• Aid,
• …

t2:

• id,
• …

So, you want to get:

And the goal is to get a row returned for every element in t1 that has
the same address as an element in t2(as well as what that address is).

Then you can do this:

SELECT
  t1.*,
  A.*
FROM t1
INNER JOIN t1_A ON t1.id = t1_A.t1id
INNER JOIN A    ON A.id  = t1_A.Aid
INNER JOIN t2_A ON a.id  = t2_A.aid
INNER JOIN t2   ON t2.id = t2_A.t2id;