How to ensure outer join with filter still returns all desired rows? - sql

Question

Imagine I have two tables in a DB like so:
products:
product_id name
----------------
1 Hat
2 Gloves
3 Shoes
sales:
product_id store_id sales
----------------------------
1 1 20
2 2 10
Now I want to do a query to list ALL products, and their sales, for store_id = 1. My first crack at it would be to use a left join, and filter to the store_id I want, or a null store_id, in case the product didn't get any sales at store_id = 1, since I want all the products listed:
SELECT name, coalesce(sales, 0)
FROM products p
LEFT JOIN sales s ON p.product_id = s.product_id
WHERE store_id = 1 or store_id is null;
Of course, this doesn't work as intended, instead I get:
name sales
---------------
Hat 20
Shoes 0
No Gloves! This is because Gloves did get sales, just not at store_id = 1, so the WHERE clause has filtered them out.
How then can I get a list of ALL products and their sales for a specific store?
Here are some queries to create the test tables:
create temp table test_products as
select 1 as product_id, 'Hat' as name;
insert into test_products values (2, 'Gloves');
insert into test_products values (3, 'Shoes');
create temp table test_sales as
select 1 as product_id, 1 as store_id, 20 as sales;
insert into test_sales values (2, 2, 10);
UPDATE: I should note that I am aware of this solution:
SELECT name, case when store_id = 1 then sales else 0 end as sales
FROM test_products p
LEFT JOIN test_sales s ON p.product_id = s.product_id;
however, it is not ideal... in reality I need to create this query for a BI tool in such a way that the tool can simply add a where clause to the query and get the desired results. Inserting the required store_id into the correct place in this query is not supported by this tool. So I'm looking for other options, if there are any.

Answer 1

Add the WHERE condition to the LEFT JOIN clause to prevent that rows go missing.
SELECT p.name, coalesce(s.sales, 0)
FROM products p
LEFT JOIN sales s ON p.product_id = s.product_id
AND s.store_id = 1;
Edit for additional request:
I assume you can manipulate the SELECT items? Then this should do the job:
SELECT p.name
,CASE WHEN s.store_id = 1 THEN coalesce(s.sales, 0) ELSE NULL END AS sales
FROM products p
LEFT JOIN sales s USING (product_id)
Also simplified the join syntax in this case.

Answer 2

I'm not near SQL, but give this a shot:
SELECT name, coalesce(sales, 0)
FROM products p
LEFT JOIN sales s ON p.product_id = s.product_id AND store_id = 1
You don't want a where on the whole query, just on your join