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Nested Sets Hierarchical Model | Evan Petersen Nested Sets Hierarchical Model | Evan Petersen http://www.evanpetersen.com/item/nested-sets.html Programming Hacks Tutorials search... Tetszik 19 6 HOME ABOUT EVAN TUTORIALS LOG IN Home Programming Nested Sets Written by Evan Petersen on Thursday, 12 April 2012. Posted in Programming, Random JUST ANOTHER HIERARCHICAL MODEL Learn how to create a hierarchical structure optimized for fetching all child nodes in a highly efficient manner. In this tutorial, we’ll look into the pros and contras of using the Nested Set model to store our hierarchical data structure. Storing Hierarchical Structures with the Nested Set Model Creating a model that can store hierarchical data is not a simple task. What do I mean by hierarchical data? Imagine that you are trying to organize products into meaningful categories. For instance, you would have a few main categories: food, appliances, and electronics. You then want to sub-divide those categories into more specific groups and then divide those categories into more specific groups. This is fairly simple to map out on paper, but not so easy to place into a data table. Such is the case when attempting to represent hierarchical data, like products and categories, in a database. One such method for modeling this type of data is called Nested Sets. This tutorial will cover the concepts behind the nested set model and when it should be implemented. I'll begin by explaining the far simpler parent child model and progressing into the nested set model. Parent-Child Model Traditionally, hierarchical structures are modeled with a strict parent to child relationship. In the case of a tree, each node will have a unique ID and a parent ID. ID numbers have no meaningful relationship to the data that is contained by the node[1] other than serving as a unique identifier. The root[2] node will have a special value as its parent ID so as to signify that you have reached the root. Consider the following table and diagram: ID parentID Value 0-1Root 10 A 20 B 31 C 41 D 53 E 62 F 1 / 9 2012.06.22. 15:32 Nested Sets Hierarchical Model | Evan Petersen http://www.evanpetersen.com/item/nested-sets.html Who am I? My name is Evan Petersen and I work as a Programmer Each arrow represents a pointer to the parent node as defined by the ID, parentID, Value table. This parent-child at Dotcomjungle in Southern Oregon. Click here to learn relationship model is one of the most widely used and understood models for representing hierarchical more about me and this structures. Each node indicates what its ID is and who it is a child of (I.e. who its parent is). website The Problem: Clinical Study Data Entry On-demand access to live Unfortunately, this model becomes cumbersome when querying for ALL children of a given node. Let’s say, for reports. Medrio saves you time and money. instance, that we want to know the ID of every child under node with ID 1. By looking at our diagram, we can www.Medrio.com quickly report that nodes 3, 4, and 5 are all children of node 1. Using our table, however, the operation isn’t Számlázzon Ingyen Nálunk Gyorsan, Könnyen, Telepítés nearly as easy. Nélkül. Több mint 3700 Cég Már Használja Szamlazz.hu/online-szamlazas Querying for Children We begin by querying for all nodes whose Recent Comments parent ID is 1. In this case, we find that nodes 3 and 4 list node 1 as their parent (the value in magnificent submit, very the red arrows signifies the query number that informative. I'm wondering why the opposite experts of this we are currently performing). We then must sector don't notice this. You query for all nodes that list 3 or 4 as their should continue your... parent. As you can see, this would get out of hand quickly on larger datasets, resulting in hundreds if not thousands of database calls so web site promotion 09. June, 2012 | # that we can retrieve all children of a single node. Note how the number of queries This comment would be to say thanks, i dont comment necessary directly corresponds to the depth typically, but when i do it really (height) of the tree. The number of queries will is usually for some thing extremely great. always be 1+ the height of the tree that you are querying. lavyneill 27. May, 2012 | # Finally! This is just what I was looknig for. Nested Sets: our knight in shining armor! This is where the concept of nested sets comes in. Bear with me as it’s not as simple as the parent child relationship we discussed earlier. Rather than have a parent ID, we will be implementing our table with left and right values. Take a look at the table Pattie and the resulting data structure: 05. May, 2012 | # You can use sleep command to pause a bash scirpt.i=1while ID Left Right Value [ "$i" != "10" ];do i=`expr $i + 1` echo number $i sleep... 0 0 13 Root 118 A 2912B Auth 04. May, 2012 | # 2 / 9 2012.06.22. 15:32 Nested Sets Hierarchical Model | Evan Petersen http://www.evanpetersen.com/item/nested-sets.html on August 7, 2006of course, it 325 C isn't *that* weird that PDO is swleor. PDO is, as far as I can 467 D see it, more of a complete database... 534 E 61011F Ceyda 04. May, 2012 | # hey, i have a test on circuit ansyials on thrusday and im having difficulties in complex Whoa, what just happened? situations, for instints. can i select a node as... Alright, let’s take a step back and look at where the next set of numbers came from. The left and right values in a node represent a set of child nodes. In turn, each child node has a left and right ismail value that represent another set of children, hence the term Nested Sets. For instance, if I wanted to retrieve all 04. May, 2012 | # children of node A (ID 1), I would run a query similar to this: dear dr. yaz z. lii am about to start teaching ntwroek analysis to ug students of electrical and electronics engineering. thanks 1 SELECT * FROM `nodes` WHERE Left > '1' AND Right<'8'; ? for... This would return C, D and E, in a single query. Notice how no recursive step was applied. In a single query, we were able to retrieve all children of A. Narasimhamurthy 04. May, 2012 | # on August 7, 2006of course, it isn't *that* weird that PDO is Well that’s great, but how do we figure out the left and right values for an slewor. PDO is, as far as I can see it, more of a complete existing tree? database... Surprisingly enough, this isn’t too tricky to accomplish. Here’s a diagram showing how I computed the left and right values of each node: Jhon 04. May, 2012 | # The parent-child data structure Determining Left and Right works wonderfully for small sets. When you start getting values into high traffic websites with deep nesting... Begin at the top left arrow, starting your count from 0. When you pass a node, increment by one and list that number Evan Petersen 18. April, 2012 | # next to the node. If you are to the left of the node, the number represents the Why not use recursive CTEs against the parent-child left value. If you’re on the right, it model? represents the right value. Keep the simple (and normalized) parent-child data structure and get the... Brandon K 18. April, 2012 | # 3 / 9 2012.06.22. 15:32 Nested Sets Hierarchical Model | Evan Petersen http://www.evanpetersen.com/item/nested-sets.html Notice how any given leaf node’s left and right values only differ by 1 while nodes with children differ by at least 3 and then in increments of two. This can tell us a lot about a given node without performing additional queries. Alternative Representation Let’s look at this data structure in a way that’s a little more meaningful with respect to left and right values: With the nodes represented in this fashion, we can very easily see how the sets are nested and the depth of a given node (distance from the root). The number line on the bottom can be thought of as the one dimensional space in which all nodes are placed. Each node consumes a distance of 1 and has a buffer of at least one slot before another node is encountered. If we were to query again for all children of node A, we can think of the query operating like so: 1 SELECT * FROM `nodes` WHERE Left > '1' AND Right<'8'; ? Inserting Nodes Let’s try inserting a new node with value G just beneath the Root node. Take a look at the resulting table and diagram below: ID Left Right Value 0 0 15 Root 118 A 2912B 325 C 467 D 534 E 4 / 9 2012.06.22. 15:32 Nested Sets Hierarchical Model | Evan Petersen http://www.evanpetersen.com/item/nested-sets.html 61011F 71314G To insert the node as a direct child of another node, you need to allocate some space in the tree. To do so, retrieve the right value of the node you wish to insert under. In this case, the right value of the root was 13 (see previous diagram). To allocate the space, add 2 to all left and right values that are greater than or equal to 13.
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