I’m certain I’m going to be embarrassed by the eventual answer, but I’ve been googling around and banging my head against the wall and can’t figure it out.
I have four tables described below. I’m trying to get the query that shows all the products related to users (through the
user_products table) AND all the categories.
CREATE TEMPORARY TABLE categories ( id INT PRIMARY KEY NOT NULL , cat_name TEXT ); CREATE TEMPORARY TABLE products ( id INT PRIMARY KEY NOT NULL , category_id INT , prod_name TEXT ); CREATE TEMPORARY TABLE users ( id INT PRIMARY KEY NOT NULL , user_name TEXT ); CREATE TEMPORARY TABLE user_products ( id INT PRIMARY KEY NOT NULL , product_id INT , user_id INT , quantity INT );
INSERT INTO categories VALUES (1, 'cat1') ,(2, 'cat2') ,(3, 'cat3') ,(4, 'cat4'); INSERT INTO products VALUES (1, 1, 'prod1') ,(2, 1, 'prod2') ,(3, 2, 'prod3') ,(4, 3, 'prod4') ,(5, 4, 'prod4'); INSERT INTO users VALUES (1, 'user1') ,(2, 'user2') ,(3, 'user3'); INSERT INTO user_products VALUES (1, 1, 1, 4) ,(2, 2, 1, 3) ,(3, 3, 1, 3) ,(4, 2, 2, 3) ,(5, 2, 2, 2) ,(6, 5, 2, 2);
I’ve tried several options, and this certainly feels like an outer join scenario. The most promising query THAT DOES NOT WORK is:
SELECT up.*, p.*, c.*, u.* FROM user_products AS up JOIN users AS u ON u.id = up.user_id JOIN products AS p ON p.id = up.product_id RIGHT OUTER JOIN categories AS c ON c.id = p.category_id WHERE u.id = 1 OR u.id IS NULL
Result from above query
id | product_id | user_id | quantity | id | category_id | prod_name | id | cat_name | id | user_name ----+------------+---------+----------+----+-------------+-----------+----+----------+----+----------- 1 | 1 | 1 | 4 | 1 | 1 | prod1 | 1 | cat1 | 1 | user1 2 | 2 | 1 | 3 | 2 | 1 | prod2 | 1 | cat1 | 1 | user1 3 | 3 | 1 | 3 | 3 | 2 | prod3 | 2 | cat2 | 1 | user1 | | | | | | | 3 | cat3 | |
The reason it does not work is that it fails to return ‘cat4’ from the categories table. Obviously this is because the
WHERE clause is selecting it out of the result set. I’m at a loss to as to get a result set that includes all categories + the details of the products and quantities for which there are records in the specified user_products table.
The query should
- return all categories
- return data populated for user info iff the user has that product (a record exists in user_products)
- not return rows with other user ids
It sounds like you are looking for a
FULL [OUTER] JOIN. Per documentation:
FULL OUTER JOIN
First, an inner join is performed. Then, for each row in T1 that does not satisfy the join condition with any row in T2, a joined row
is added with null values in columns of T2. Also, for each row of T2
that does not satisfy the join condition with any row in T1, a joined
row with null values in the columns of T1 is added.
SELECT * FROM users u JOIN user_products up ON u.id = up.user_id AND u.id = 1 JOIN products p ON p.id = up.product_id FULL JOIN categories c ON c.id = p.category_id;
This returns all categories and also returns all user-product combinations for the given
If you are enforcing referential integrity with foreign keys and
products.category_id is defined
NOT NULL, so that every product is assigned to an existing category (not in your question), you can replace the
FULL JOIN with a
Plus, either way, the condition to select a specific user has to move to a subquery or (simpler) to a
JOIN condition between the first three tables. It has to be applied before the last table
categories is joined.
I believe you want to show all categories.
If so, you will need to make all of your joins RIGHT OUTER JOINS.
- Associativity and precedence for outer join operators is poorly defined and unpredictable
- You need to keep going ‘outer’ once you’ve started. Because you’re using a RIGHT join, each preceding join must also be a RIGHT join. If not, the NULLs in the outer table will never join out to product-user or user tables, and this will cut out your categories as well
Note you will likely see users repeat once you get this working, because users can be associated with multiple products in multiple categories.