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POMysql Using Result of Subquery in Main Query
 

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    <p>Basically, what I want to do is to get all children in another table within a certain birth year. So I have two tables. Let's say it's a school table</p> <p><strong>School</strong></p> <pre><code>school_id child_id </code></pre> <p><strong>Children</strong></p> <pre><code>child_id birth_year name etc </code></pre> <p>My first attempt is to use subquery, which is something like this</p> <pre><code>SELECT *, (SELECT COUNT(*) FROM school LEFT JOIN children ON school.child_id = children.child_id) as total FROM school LEFT JOIN children ON school.child_id = children.child_id GROUP BY birth_year </code></pre> <p>The problem with this query is the subquery will run all throughout the records, so if I have 1000 records, I think the query (and the subquery) will run 1000 times before grouping by birth_year, which is slow, it's almost 3-5 seconds for 500 sample data.</p> <p>So to optimize it, I'm doing this.</p> <p><strong>Recursive Query Using PHP</strong></p> <p>First, get the distinct birth year of ALL children who are in school. So I'm gonna query something like</p> <pre><code>SELECT birth_year FROM school LEFT JOIN children ON school.child_id = children.child_id </code></pre> <p>It will return data like</p> <pre><code>birth_year ========== 2009 2010 2011 </code></pre> <p>Which I'm gonna use in another query in PHP (let's say I store the result in $row variable)</p> <pre><code>foreach ($row as $r){ $new_array[] = count($this-&gt;db-&gt;get_child_data($r-&gt;birth_year)); //this is pseudocode only, to get the number of children data who have birth_year of 2009-2011 } </code></pre> <p>Though it will run additional three queries, this is really fast as the count is simple. It only takes less than 0.5 seconds for 500 sample data.</p> <p>However, I'm wondering if there's any way of optimizing it? Or better, is there a way to do it in a single query with similar performance?</p> <p>I'm trying to do this, but it ends up super slow and crashes my WAMP.</p> <pre><code>SELECT * FROM children WHERE birth_year IN (SELECT GROUP_CONCAT(DISTINCT birth_year) FROM school LEFT JOIN children ON school.child_id = children.child_id ) </code></pre> <p>The subquery </p> <pre><code>SELECT GROUP_CONCAT(DISTINCT birth_year) FROM school LEFT JOIN children ON school.child_id = children.child_id </code></pre> <p>when run separately correctly and quickly returns</p> <pre><code>2009,2010,2011 </code></pre> <p>And when I query</p> <pre><code>SELECT * FROM children WHERE birth_year IN (2009,2010,2011) </code></pre> <p>It also works fast , so I'm quite confused why when I join both queries, it is slow to the point to crash my WAMP.</p> <p>Sorry for the long post and thanks in advance</p>
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