mysql inner join 3 tables and union - mysql

Question

I'm used to make queries with sparql, but i have to do this one with mysql that i don't really know the syntax.
I have these three tables:
Products:
id o
1 p1
2 p2
Specification:
id o
3 sp1
4 sp2
Source:
id o
1 s1
3 s1
2 s2
4 s2
As we see, the products and the specifications can be connected with the source different ids, so i guess i have to make 2 selects with 2 inner joins between source > product and source > specification and an inner join between the 2 selects :
SELECT * FROM
(
SELECT pt.o as po, st.id as psID, st.o as pso
FROM source_test st
inner join products_test pt on st.id = pt.id
) x INNER JOIN
(
SELECT st2.o as spo, st1.id as spsID,st1.o as spso
FROM source_test st1
inner join specification_test st2 on st1.id =st2.id
) y
This gives:
po psID pso spo spsID spso
p1 1 s1 sp1 3 s1
p2 2 s2 sp1 3 s1
p1 1 s1 sp2 4 s2
p2 2 s2 sp2 4 s2
Now i need to filter product (po) and the specification (spo) that have the same source (pso),(spso)
I'm i on doing things the right way, what could be the continuation or a good query to get:
po spo spso
p1 sp1 s1
p2 sp1 s2
Thanks in advance.

Answer 1

select p.o, s.o, so.o
from products p
left join specification s
left join source so on p.id=s.id and p.id = so.id;

Answer 2

The only thing i had to do is to to filter y ON x.pso = y.spso
SELECT * FROM
(
SELECT product.o as po, source.s as psID, source.o as pso FROM source
inner join product
on source.s = product.s
) x INNER JOIN
(
SELECT specification.o as spo, source.s as spsID, source.o as spso FROM source
inner join specification
on source.s = specification.s
) y ON x.pso = y.spso
result:
po psID pso spo spsID spso
p1 1 s1 sp1 3 s1
p2 2 s2 sp2 4 s2

Answer 3

Add ON x.pso = y.spso to your query:
SELECT x.po, y.spo, y.spso
FROM
(
SELECT products_test.o as po, source_test.id as psID, source_test.o as pso FROM source_test
inner join products_test
on source_test.id = products_test.id
) x INNER JOIN
(
SELECT specification_test.o as spo, source_test.id as spsID,source_test.o as spso
FROM source_test
inner join specification_test on source_test.id =specification_test.id
) y
ON x.pso = y.spso