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POHierarchical table - how to get paths of the items [linked lists in MySQL]
 

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  1. POHierarchical table - how to get paths of the items [linked lists in MySQL]
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    <p>I have a hierarchical table in MySQL: <code>parent</code> field of each item points to the <code>id</code> field of its parent item. For each item I can get the list of all its parents [regardless the depth] using the <a href="http://explainextended.com/2009/07/20/hierarchical-data-in-mysql-parents-and-children-in-one-query/" rel="nofollow noreferrer">query described here</a>. With <code>GROUP_CONCAT</code> I get the full path as a single string:</p> <pre><code>SELECT GROUP_CONCAT(_id SEPARATOR ' &gt; ') FROM ( SELECT @r AS _id, ( SELECT @r := parent FROM t_hierarchy WHERE id = _id ) AS parent, @l := @l + 1 AS lvl FROM ( SELECT @r := 200, @l := 0 ) vars, t_hierarchy h WHERE @r &lt;&gt; 0 ORDER BY lvl DESC ) x </code></pre> <p>I can make this work only if the <code>id</code> of the item is fixed [it's <code>200</code> in this case].</p> <p>I want to do the same for all rows: retrieve the whole table with one additional field (<code>path</code>) which will display the full path. The only solution that comes to my mind is to wrap this query in another select, set a temporary variable <code>@id</code> and use it inside the subquery. But it doesn't work. I get <code>NULL</code>s in the <code>path</code> field.</p> <pre><code>SELECT @id := id, parent, ( SELECT GROUP_CONCAT(_id SEPARATOR ' &gt; ') FROM ( SELECT @r AS _id, ( SELECT @r := parent FROM t_hierarchy WHERE id = _id ) AS parent, @l := @l + 1 AS lvl FROM ( SELECT @r := @id, @l := 0 ) vars, t_hierarchy h WHERE @r &lt;&gt; 0 ORDER BY lvl DESC ) x ) as path FROM t_hierarchy </code></pre> <p><strong>P.S.</strong> I know I can store the paths in a separate field and update them when inserting/updating, but I need a solution based on the <em>linked list technique</em>.</p> <p><strong>UPDATE:</strong> I would like to see a solution that will not use recursion or constructs like <code>for</code> and <code>while</code>. The above method for finding paths doesn't use any loops or functions. I want to find a solution in the same logic. Or, if it's impossible, please try to explain why!</p>
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    1. USHrant Khachatrian
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