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POMySQL query to determine records that are related to category A and B
 

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    <p>Here is a simplified version of my data:</p> <pre><code>products: +----+-----------+ | id | name | +----+-----------+ | 1 | Product X | | 2 | Product Y | | 3 | Product Z | +----+-----------+ categories: +----+---------------+ | id | name | +----+---------------+ | 1 | Hotel | | 2 | Accommodation | +----+---------------+ category_product +----+------------+-------------+ | id | product_id | category_id | +----+------------+-------------+ | 1 | 1 | 1 | | 2 | 1 | 2 | | 3 | 2 | 1 | | 4 | 3 | 2 | +----+------------+-------------+ </code></pre> <p>How do I construct an efficient query that will only retrieve <code>products</code> that have <strong>both</strong> categories "Hotel" and "Accommodation" related (eg. Product X)?</p> <p>I first tried a join approach</p> <pre class="lang-sql prettyprint-override"><code>SELECT * FROM products p JOIN category_product cp ON p.id = cp.product_id WHERE cp.category_id = 1 OR cp.category_id = 2 </code></pre> <p>^ This doesn't work because it doesn't contrain the query to <strong>containing both</strong>.</p> <p>I have found an approach using sub-queries that works... but I've been warned against sub-queries for performance reasons:</p> <pre class="lang-sql prettyprint-override"><code>SELECT * FROM products p WHERE ( SELECT id FROM category_product WHERE product_id = p.id AND category_id = 1 ) AND ( SELECT id FROM category_product WHERE product_id = p.id AND category_id = 2 ) </code></pre> <p>Are there any better solutions (or how about alternatives)? I've considered de-normalizing categories to an extra column on products but would ideally like to avoid that. Hoping for a magic bullet solution!</p> <p><strong>UPDATE</strong></p> <p>I've run some of the (great) solutions provided in the answers: My data is 235 000 category_product rows and 58 000 products and obviously benchmarks are always dependent on environment and indexes etc.</p> <p>"Relational division" @podiluska</p> <pre><code>2 categories: 2826 rows ~ 20ms 5 categories: 46 rows ~ 25-30 ms 8 categories: 1 rows ~ 25-30 ms </code></pre> <p>"Where exists" @Tim Schmelter</p> <pre><code>2 categories: 2826 rows ~ 5-7ms 5 categories: 46 rows ~ 30 ms 8 categories: 1 rows ~ 300 ms </code></pre> <p>One can see the results start to diverge with having a greater number of categories thrown in. I'll look at using "relational division" as it provides consistent results but implementation might cause me to look at "where exists" too (long format <a href="http://pastebin.com/6NRX0QbJ" rel="nofollow">http://pastebin.com/6NRX0QbJ</a>)</p>
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