Transitive joins in postgresql

Question

What is the best way to achieve a transitive join in postgresql? Currently, I want to do a full outer join for tables, a, b, and c. My query currently looks like this:

SELECT *
FROM a 
FULL OUTER JOIN b
ON a."ID" = b."ID"
FULL OUTER JOIN c
ON a."ID" = c."ID"

I'm running into an issue where some records that match in table B and table C are showing up in different rows in the query output, and I realized that it must be because I have not explicitly joined tables B and C. What is the best way to write a "transitive" query where a=b, a=c, and b=c?
Here is an example of my current output. Right now, when a matching ID exists for just tables B and C, I get 2 different rows:

A ID	B ID	C ID
32	32	null
35	35	35
36	null	36
null	42	null
null	null	42

Here is my desired output:

A ID	B ID	C ID
32	32	null
35	35	35
36	null	36
null	42	42

Answer 1

Use using:

SELECT *
FROM a FULL OUTER JOIN
     b
     USING ("ID") FULL OUTER JOIN
     c
     USING ("ID");

If in your real example, the columns have different names:

SELECT *
FROM a FULL OUTER JOIN
     b
     ON b.id = a.id FULL OUTER JOIN
     c
     ON c.id = COALESCE(b.id, a.id);