When JOINing 3 tables in Postgres, can the results be sorted by values in the 2 joined tables? - postgresql

Question

I have a database with three tables. Ultimately I want to JOIN the three tables and sort them by a column shared by two of the tables.
A main item table with foreign keys (product_id) to the two sub-tables:
items
CREATE TABLE items (
id INT NOT NULL,
product_id varchar(40) NOT NULL,
type CHAR NOT NULL
);
and then a table corresponding to each typeA and typeB. They have differing columns, but for the sake of this exercise I'm only including the columns they have in common:
CREATE TABLE products_a (
id varchar(40) NOT NULL,
name varchar(40) NOT NULL,
price INT NOT NULL
);
CREATE TABLE products_b (
id varchar(40) NOT NULL,
name varchar(40) NOT NULL,
price INT NOT NULL
);
Some example rows:
INSERT INTO items VALUES
( 1, 'abc', 'a' ),
( 2, 'def', 'b' ),
( 3, 'ghi', 'a' ),
( 4, 'jkl', 'b' );
INSERT INTO products_a VALUES
( 'abc', 'product 1', 10 ),
( 'ghi', 'product 2', 50 );
INSERT INTO products_b VALUES
( 'def', 'product 3', 20 ),
( 'jkl', 'product 4', 100 );
I have a JOIN working, but my sorting is not interpolating the rows as I would expect.
Query:
SELECT
items.id AS item_id,
products_a.name AS product_a_name,
products_a.price AS product_a_price,
products_b.name AS product_b_name,
products_b.price AS product_b_price
FROM items
FULL JOIN products_a ON items.product_id = products_a.id
FULL JOIN products_b ON items.product_id = products_b.id
ORDER BY 3, 5 ASC;
Actual result:
item_id
product_a_name
product_a_price
product_b_name
product_b_price
1
product 1
10
NULL
NULL
3
product 2
50
NULL
NULL
2
NULL
NULL
product 3
20
4
NULL
NULL
product 4
100
Desired result:
item_id
product_a_name
product_a_price
product_b_name
product_b_price
1
product 1
10
NULL
NULL
2
NULL
NULL
product 3
20
3
product 2
50
NULL
NULL
4
NULL
NULL
product 4
100
I realize this is a weird table setup, but simplified this way looks more contrived than it is. Ultimately the sorting matches the real use case, though, and changing the DB schema is not an option. I feel like I am missing something simple here, just sorting by either one column or another. Any help is appreciated.

Answer 1

Use COALESCE in the ORDER BY clause to always sort by the first non NULL price:
SELECT
items.id AS item_id,
products_a.name AS product_a_name,
products_a.price AS product_a_price,
products_b.name AS product_b_name,
products_b.price AS product_b_price
FROM items
FULL JOIN products_a ON items.product_id = products_a.id
FULL JOIN products_b ON items.product_id = products_b.id
ORDER BY
COALESCE(3, 5);