Postgresql : How to join with multiple cross-reference table? - postgresql

Question

I've seen a lot of post on multiple JOIN but it didn't help me in my case.
Consider that I have three tables and two cross-reference tables. That's the difference with the others posts where they had multiple tables but one cross-reference table in the FROM.
Table1 -> cross-ref1 <- table2 -> cross-ref2 <- table3
My version of Postgresql is : 9.0.11, and I'm working with W7 64 bits.
My request is the following stuff :
Select [columns] from cross-ref1, cross-ref2
INNER JOIN table1 ON table1.id_table1=cross-ref1.ref_id_table1
INNER JOIN table2 ON table2.id=cross-ref1.ref_id_table2
INNER JOIN table2 On table2.id_table2=cross-ref2.ref_id_table2
INNER JOIN table3 ON table3.id_table3=cross-ref2.ref_id_table3
The error message is : "Table name is specified more than once."
Can you explain me the error ?
Thanks

Answer 1

Cross-reference tables need separate columns for each side of the reference. An xref table with just one column makes no sense, as it can only refer to rows with the same ID on each side.
A typical setup would be:
CREATE TABLE a (
id integer primary key,
avalue text not null
);
CREATE TABLE b (
id integer primary key,
bvalue text not null
);
CREATE TABLE ab (
a_id integer references a(id),
b_id integer references b(id),
PRIMARY KEY(a_id, b_id)
);
Given sample data:
INSERT INTO a(id, avalue) VALUES
(1, 'a1'), (2, 'a2'), (3, 'a3'), (4, 'a4');
INSERT INTO b(id, bvalue) VALUES
(41, 'b1'), (42, 'b2'), (43, 'b3');
INSERT INTO ab(a_id, b_id) VALUES
(1, 41), (1, 42), (2, 43);
You'd find the pairings of a and b with:
SELECT avalue, bvalue
FROM a
INNER JOIN ab ON (a.id = ab.a_id)
INNER JOIN b ON (b.id = ab.b_id);
The crucial thing here is that you're joining on ab.a_id on the a side, and ab.b_id on the b side. Observe demo here: http://sqlfiddle.com/#!12/3228a/1
This is pretty much "many-to-many table relationships 101", so it might be worth doing some more study of introductory SQL and relational database tutorials and documentation.

Answer 2

You can't use the same table name twice (table2). In this case you need to use aliases like t1, t2a, t2b, ...
SELECT
...
FROM
table1 AS t1
INNER JOIN table2 AS t2a
ON t2a.id= ...
INNER JOIN table2 AS t2b
ON t2b.id= ...
INNER JOIN table3 AS t3
ON t3.id= ...
...
Now you can join whatever you want, how many times you want etc.

Answer 3

You have to explain what result you want to have. For example the following SQL is valid from syntax point of view, not sure about business point of view:
-- this will create sample data with 5 tables
with
crossref1(ref_id) as (VALUES (1),(2),(3)),
crossref2 (ref_id) as (VALUES (2),(3),(4)),
table1 (ref_id) as (VALUES (3),(4),(5)),
table2 (ref_id) as (VALUES (1),(2),(3)),
table3 (ref_id) as (VALUES (1),(2),(3))
-- valid SQL based on your example
select * from
crossref1
cross join crossref2
join table1 on table1.ref_id=crossref1.ref_id
join table2 as t2_1 on t2_1.ref_id=crossref1.ref_id
join table2 as t2_2 on t2_2.ref_id=crossref2.ref_id
join table3 on table3.ref_id=crossref2.ref_id
With your SQL there are two problems:
You have two references to table2, you have to add alias
You have to use cross join syntax instead of ,
If you would like to understand how with works (how I created sample data), PostgreSQL has excellent documentation on this.