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POHow to ensure outer join with filter still returns all desired rows?
 

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  1. POHow to ensure outer join with filter still returns all desired rows?
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    <p>Imagine I have two tables in a DB like so:</p> <pre><code>products: product_id name ---------------- 1 Hat 2 Gloves 3 Shoes sales: product_id store_id sales ---------------------------- 1 1 20 2 2 10 </code></pre> <p>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:</p> <pre><code>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; </code></pre> <p>Of course, this doesn't work as intended, instead I get:</p> <pre><code>name sales --------------- Hat 20 Shoes 0 </code></pre> <p>No Gloves! This is because Gloves did get sales, just not at store_id = 1, so the WHERE clause has filtered them out.</p> <p>How then can I get a list of ALL products and their sales for a specific store?</p> <p>Here are some queries to create the test tables:</p> <pre><code>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); </code></pre> <p>UPDATE: I should note that I am aware of this solution:</p> <pre><code>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; </code></pre> <p>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.</p>
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