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    <p>The <a href="http://en.wikipedia.org/wiki/Entity-attribute-value_model" rel="nofollow noreferrer">entity-attribute-value</a> model that you suggest could fit in this scenario.</p> <p>Regarding the filtering query, you have to understand that with the EAV model you will sacrifice plenty of query power, so this can become quite tricky. However this one way to tackle your problem:</p> <pre><code>SELECT stuff.id FROM stuff JOIN (SELECT COUNT(*) matches FROM table WHERE (`key` = X1 AND `value` = V1) OR (`key` = X2 AND `value` = V2) GROUP BY id ) sub_t ON (sub_t.matches = 2 AND sub_t.id = stuff.id) GROUP BY stuff.id; </code></pre> <p>One inelegant feature of this approach is that you need to specify the number of attribute/value pairs that you expect to match in <code>sub_t.matches = 2</code>. If we had three conditions we would have had to specify <code>sub_t.matches = 3</code>, and so on.</p> <p>Let's build a test case:</p> <pre><code>CREATE TABLE stuff (`id` varchar(20), `key` varchar(20), `value` varchar(20)); INSERT INTO stuff VALUES ('apple', 'color', 'red'); INSERT INTO stuff VALUES ('mango', 'color', 'yellow'); INSERT INTO stuff VALUES ('banana', 'color', 'yellow'); INSERT INTO stuff VALUES ('apple', 'taste', 'sweet'); INSERT INTO stuff VALUES ('mango', 'taste', 'sweet'); INSERT INTO stuff VALUES ('banana', 'taste', 'bitter-sweet'); INSERT INTO stuff VALUES ('apple', 'origin', 'US'); INSERT INTO stuff VALUES ('mango', 'origin', 'MEXICO'); INSERT INTO stuff VALUES ('banana', 'origin', 'US'); </code></pre> <p>Query:</p> <pre><code>SELECT stuff.id FROM stuff JOIN (SELECT COUNT(*) matches, id FROM stuff WHERE (`key` = 'color' AND `value` = 'yellow') OR (`key` = 'taste' AND `value` = 'sweet') GROUP BY id ) sub_t ON (sub_t.matches = 2 AND sub_t.id = stuff.id) GROUP BY stuff.id; </code></pre> <p>Result:</p> <pre><code>+-------+ | id | +-------+ | mango | +-------+ 1 row in set (0.02 sec) </code></pre> <p>Now let's insert another fruit with <code>color=yellow</code> and <code>taste=sweet</code>:</p> <pre><code>INSERT INTO stuff VALUES ('pear', 'color', 'yellow'); INSERT INTO stuff VALUES ('pear', 'taste', 'sweet'); INSERT INTO stuff VALUES ('pear', 'origin', 'somewhere'); </code></pre> <p>The same query would return:</p> <pre><code>+-------+ | id | +-------+ | mango | | pear | +-------+ 2 rows in set (0.00 sec) </code></pre> <p>If we want to restrict this result to entities with <code>origin=MEXICO</code>, we would have to add another <code>OR</code> condition and check for <code>sub_t.matches = 3</code> instead of <code>2</code>.</p> <pre><code>SELECT stuff.id FROM stuff JOIN (SELECT COUNT(*) matches, id FROM stuff WHERE (`key` = 'color' AND `value` = 'yellow') OR (`key` = 'taste' AND `value` = 'sweet') OR (`key` = 'origin' AND `value` = 'MEXICO') GROUP BY id ) sub_t ON (sub_t.matches = 3 AND sub_t.id = stuff.id) GROUP BY stuff.id; </code></pre> <p>Result:</p> <pre><code>+-------+ | id | +-------+ | mango | +-------+ 1 row in set (0.00 sec) </code></pre> <p>As in every approach, there are certain advantages and disadvantages when using the EAV model. Make sure you research the topic extensively in the context of your application. You may even want to consider an alternative relational databases, such as <a href="http://cassandra.apache.org/" rel="nofollow noreferrer">Cassandra</a>, <a href="http://couchdb.apache.org/" rel="nofollow noreferrer">CouchDB</a>, <a href="http://www.mongodb.org/" rel="nofollow noreferrer">MongoDB</a>, <a href="http://project-voldemort.com/" rel="nofollow noreferrer">Voldemort</a>, <a href="http://hadoop.apache.org/hbase/" rel="nofollow noreferrer">HBase</a>, <a href="http://aws.amazon.com/simpledb/" rel="nofollow noreferrer">SimpleDB</a> or other key-value stores.</p>
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    1. POHow to implement filter system in SQL?
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    1. USDaniel Vassallo
    1. USDaniel Vassallo
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    1. COWow this looks pretty complicated. Thanks for the solution. There are a lot of people opposing to me using this design, so for now I am seriously considering whether I should use the EVA model
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    2. CO@sadvaw: The opposition mainly stems from the fact that when you use the EAV model in a relational database, it is like using a van to carry yourself around the city: therefore you're not using it for what it was built to do. However it can still be done, and the feasibility of such often depends on the scale (how much you do it, or how big). Therefore I would say that if all you are doing in the database is this, then I would actually consider alternatives to an RDBMS. However if you have a larger database and this is just a small part, then these considerations could be less important.
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