Database Structure for Tree Data Structure

Question

What would be the best way to implement a customizable tree data structure   meaning  a tree structure with an unknown number of level  in a database  I ve done this once before using a table with a foreign key to itself  What other implementations could you see  and does this implementation make sense

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If anyone using MS SQL Server 2008 and higher lands on this question  SQL Server 2008 and higher has a new  hierarchyId  feature designed specifically for this task   More info at https   docs microsoft com en-us sql relational-databases hierarchical-data-sql-server

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Having a table with a foreign key to itself does make sense to me    You can then use a common table expression in SQL or the connect by prior statement in Oracle to build your tree

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If you have to use Relational DataBase to organize tree data structure then Postgresql has cool ltree module that provides data type for representing labels of data stored in a hierarchical tree-like structure  You can get the idea from there  For more information see  http   www postgresql org docs 9 0 static ltree html   In common LDAP is used to organize records in hierarchical structure

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Have a look at Managing Hierarchical Data in MySQL  It discusses two approaches for storing and managing hierarchical  tree-like  data in a relational database   The first approach is the adjacency list model  which is what you essentially describe  having a foreign key that refers to the table itself  While this approach is simple  it can be very inefficient for certain queries  like building the whole tree   The second approach discussed in the article is the nested set model  This approach is far more efficient and flexible  Refer to the article for detailed explanation and example queries

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You mention the most commonly implemented  which is Adjacency List  https   blogs msdn microsoft com mvpawardprogram 2012 06 25 hierarchies-convert-adjacency-list-to-nested-sets  There are other models as well  including materialized path and nested sets  http   communities bmc com communities docs DOC-9902  Joe Celko has written a book on this subject  which is a good reference from a general SQL perspective  it is mentioned in the nested set article link above    Also  Itzik Ben-Gann has a good overview of the most common options in his book  Inside Microsoft SQL Server 2005  T-SQL Querying    The main things to consider when choosing a model are   1  Frequency of structure change - how frequently does the actual structure of the tree change  Some models provide better structure update characteristics  It is important to separate structure changes from other data changes however  For example  you may want to model a company s organizational chart  Some people will model this as an adjacency list  using the employee ID to link an employee to their supervisor  This is usually a sub-optimal approach  An approach that often works better is to model the org structure separate from employees themselves  and maintain the employee as an attribute of the structure  This way  when an employee leaves the company  the organizational structure itself does not need to be changes  just the association with the employee that left   2  Is the tree write-heavy or read-heavy - some structures work very well when reading the structure  but incur additional overhead when writing to the structure   3  What types of information do you need to obtain from the structure - some structures excel at providing certain kinds of information about the structure  Examples include finding a node and all its children  finding a node and all its parents  finding the count of child nodes meeting certain conditions  etc  You need to know what information will be needed from the structure to determine the structure that will best fit your needs

[sql] Database Structure for Tree Data Structure

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