What are the options for storing hierarchical data in a relational database

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Good Overviews  Generally speaking  you re making a decision between fast read times  for example  nested set  or fast write times  adjacency list   Usually  you end up with a combination of the options below that best fit your needs  The following provides some in-depth reading    One more Nested Intervals vs  Adjacency List comparison  the best comparison of Adjacency List  Materialized Path  Nested Set and Nested Interval I ve found  Models for hierarchical data  slides with good explanations of tradeoffs and example usage Representing hierarchies in MySQL  very good overview of Nested Set in particular Hierarchical data in RDBMSs  most comprehensive and well-organized set of links I ve seen  but not much in the way of explanation   Options  Ones I am aware of and general features    Adjacency List    Columns  ID  ParentID Easy to implement  Cheap node moves  inserts  and deletes  Expensive to find the level  ancestry  amp  descendants  path Avoid N 1 via Common Table Expressions in databases that support them  Nested Set  a k a Modified Preorder Tree Traversal    Columns  Left  Right Cheap ancestry  descendants Very expensive O n 2  moves  inserts  deletes due to volatile encoding  Bridge Table  a k a  Closure Table  w triggers    Uses separate join table with  ancestor  descendant  depth  optional  Cheap ancestry and descendants Writes costs O log n   size of subtree  for insert  updates  deletes Normalized encoding  good for RDBMS statistics  amp  query planner in joins Requires multiple rows per node  Lineage Column  a k a  Materialized Path  Path Enumeration    Column  lineage  e g   parent child grandchild etc     Cheap descendants via prefix query  e g  LEFT lineage         enumerated path   Writes costs O log n   size of subtree  for insert  updates  deletes Non-relational  relies on Array datatype or serialized string format  Nested Intervals   Like nested set  but with real float decimal so that the encoding isn t volatile  inexpensive move insert delete  Has real float decimal representation precision issues Matrix encoding variant adds ancestor encoding  materialized path  for  free   but with added trickiness of linear algebra   Flat Table   A modified Adjacency List that adds a Level and Rank  e g  ordering  column to each record  Cheap to iterate paginate over Expensive move and delete Good Use  threaded discussion - forums   blog comments  Multiple lineage columns   Columns  one for each lineage level  refers to all the parents up to the root  levels down from the item s level are set to NULL Cheap ancestors  descendants  level Cheap insert  delete  move of the leaves Expensive insert  delete  move of the internal nodes Hard limit to how deep the hierarchy can be    Database Specific Notes  MySQL   Use session variables for Adjacency List   Oracle   Use CONNECT BY to traverse Adjacency Lists   PostgreSQL   ltree datatype for Materialized Path   SQL Server   General summary 2008 offers HierarchyId data type appears to help with Lineage Column approach and expand the depth that can be represented

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If your database supports arrays  you can also implement a lineage column or materialized path as an array of parent ids   Specifically with Postgres you can then use the set operators to query the hierarchy  and get excellent performance with GIN indices   This makes finding parents  children  and depth pretty trivial in a single query   Updates are pretty manageable as well   I have a full write up of using arrays for materialized paths if you re curious

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This design was not mentioned yet  Multiple lineage columns Though it has limitations  if you can bear them  it s very simple and very efficient  Features   Columns  one for each lineage level  refers to all the parents up to the root  levels below the current items  level are set to 0  or NULL  There is a fixed limit to how deep the hierarchy can be Cheap ancestors  descendants  level Cheap insert  delete  move of the leaves Expensive insert  delete  move of the internal nodes  Here follows an example - taxonomic tree of birds so the hierarchy is Class Order Family Genus Species - species is the lowest level  1 row   1 taxon  which corresponds to species in the case of the leaf nodes   CREATE TABLE  taxons       TaxonId  smallint 6  NOT NULL default  0      ClassId  smallint 6  default NULL     OrderId  smallint 6  default NULL     FamilyId  smallint 6  default NULL     GenusId  smallint 6  default NULL     Name  varchar 150  NOT NULL default        and the example of the data   --------- --------- --------- ---------- --------- -------------------------------    TaxonId   ClassId   OrderId   FamilyId   GenusId   Name                             --------- --------- --------- ---------- --------- -------------------------------        254         0         0          0         0   Aves                                  255       254         0          0         0   Gaviiformes                           256       254       255          0         0   Gaviidae                              257       254       255        256         0   Gavia                                 258       254       255        256       257   Gavia stellata                        259       254       255        256       257   Gavia arctica                         260       254       255        256       257   Gavia immer                           261       254       255        256       257   Gavia adamsii                         262       254         0          0         0   Podicipediformes                      263       254       262          0         0   Podicipedidae                         264       254       262        263         0   Tachybaptus                      This is great because this way you accomplish all the needed operations in a very easy way  as long as the internal categories don t change their level in the tree

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This is a very partial answer to your question  but I hope still useful   Microsoft SQL Server 2008 implements two features that are extremely useful for managing hierarchical data    the HierarchyId data type  common table expressions  using the with keyword    Have a look at  Model Your Data Hierarchies With SQL Server 2008  by Kent Tegels on MSDN for starts  See also my own question  Recursive same-table query in SQL Server 2008

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I am using PostgreSQL with closure tables for my hierarchies  I have one universal stored procedure for the whole database   CREATE FUNCTION nomen tree   RETURNS trigger     LANGUAGE plpgsql     AS     DECLARE   old parent INTEGER    new parent INTEGER    id nom INTEGER    txt name TEXT  BEGIN -- TG ARGV 0    name of table with entities with PARENT-CHILD relationships  TBL ORIG  -- TG ARGV 1    name of helper table with ANCESTOR  CHILD  DEPTH information  TBL TREE  -- TG ARGV 2    name of the field in TBL ORIG which is used for the PARENT-CHILD relationship  FLD PARENT      IF TG OP    INSERT  THEN     EXECUTE  INSERT INTO      TG ARGV 1        child id ancestor id depth           SELECT  1 id  1 id 0 UNION ALL       SELECT  1 id ancestor id depth 1 FROM      TG ARGV 1       WHERE child id  1      TG ARGV 2  USING NEW      ELSE                                                                -- EXECUTE does not support conditional statements inside     EXECUTE  SELECT  1      TG ARGV 2        2      TG ARGV 2  INTO old parent new parent USING OLD NEW      IF COALESCE old parent 0   lt  gt  COALESCE new parent 0  THEN       EXECUTE         -- prevent cycles in the tree       UPDATE      TG ARGV 0       SET      TG ARGV 2          1      TG ARGV 2               WHERE id  2      TG ARGV 2       AND EXISTS SELECT 1 FROM              TG ARGV 1       WHERE child id  2      TG ARGV 2       AND ancestor id  2 id         -- first remove edges between all old parents of node and its descendants       DELETE FROM      TG ARGV 1       WHERE child id IN          SELECT child id FROM      TG ARGV 1       WHERE ancestor id    1 id          AND ancestor id IN          SELECT ancestor id FROM      TG ARGV 1       WHERE child id    1 id AND ancestor id  lt  gt   1 id         -- then add edges for all new parents           INSERT INTO      TG ARGV 1        child id ancestor id depth           SELECT child id ancestor id d c d a FROM          SELECT child id depth AS d c FROM      TG ARGV 1       WHERE ancestor id  2 id  AS child         CROSS JOIN          SELECT ancestor id depth 1 AS d a FROM      TG ARGV 1       WHERE child id  2               TG ARGV 2        AS parent   USING OLD  NEW      END IF    END IF    RETURN NULL  END         Then for each table where I have a hierarchy  I create a trigger  CREATE TRIGGER nomenclature tree tr AFTER INSERT OR UPDATE ON nomenclature FOR EACH ROW EXECUTE PROCEDURE nomen tree  my db nomenclature    my db nom helper    parent id      For populating a closure table from existing hierarchy I use this stored procedure   CREATE FUNCTION rebuild tree tbl base text  tbl closure text  fld parent text  RETURNS void     LANGUAGE plpgsql     AS    BEGIN     EXECUTE  TRUNCATE      tbl closure           INSERT INTO      tbl closure       child id ancestor id depth           WITH RECURSIVE tree AS                 SELECT id AS child id id AS ancestor id 0 AS depth FROM      tbl base              UNION ALL          SELECT t id ancestor id depth 1 FROM      tbl base      AS t         JOIN tree ON child id        fld parent                    SELECT   FROM tree    END        Closure tables are defined with 3 columns - ANCESTOR ID  DESCENDANT ID  DEPTH  It is possible  and I even advice  to store records with same value for ANCESTOR and DESCENDANT  and a value of zero for DEPTH  This will simplify the queries for retrieval of the hierarchy  And they are very simple indeed   -- get all descendants SELECT tbl orig   depth FROM tbl closure LEFT JOIN tbl orig ON descendant id   tbl orig id WHERE ancestor id   XXX AND depth  lt  gt  0  -- get only direct descendants SELECT tbl orig   FROM tbl closure LEFT JOIN tbl orig ON descendant id   tbl orig id WHERE ancestor id   XXX AND depth   1  -- get all ancestors SELECT tbl orig   FROM tbl closure LEFT JOIN tbl orig ON ancestor id   tbl orig id WHERE descendant id   XXX AND depth  lt  gt  0  -- find the deepest level of children SELECT MAX depth  FROM tbl closure WHERE ancestor id   XXX

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My favorite answer is as what the first sentence in this thread suggested   Use an Adjacency List to maintain the hierarchy and use Nested Sets to query the hierarchy   The problem up until now has been that the coversion method from an Adjacecy List to Nested Sets has been frightfully slow because most people use the extreme RBAR method known as a  Push Stack  to do the conversion and has been considered to be way to expensive to reach the Nirvana of the simplicity of maintenance by the Adjacency List and the awesome performance of Nested Sets   As a result  most people end up having to settle for one or the other especially if there are more than  say  a lousy 100 000 nodes or so   Using the push stack method can take a whole day to do the conversion on what MLM ers would consider to be a small million node hierarchy   I thought I d give Celko a bit of competition by coming up with a method to convert an Adjacency List to Nested sets at speeds that just seem impossible  Here s the performance of the push stack method on my i5 laptop   Duration for     1 000 Nodes   00 00 00 870  Duration for    10 000 Nodes   00 01 01 783  70 times slower instead of just 10  Duration for   100 000 Nodes   00 49 59 730  3 446 times slower instead of just 100   Duration for 1 000 000 Nodes    Didn t even try this    And here s the duration for the new method  with the push stack method in parenthesis    Duration for     1 000 Nodes   00 00 00 053  compared to 00 00 00 870  Duration for    10 000 Nodes   00 00 00 323  compared to 00 01 01 783  Duration for   100 000 Nodes   00 00 03 867  compared to 00 49 59 730  Duration for 1 000 000 Nodes   00 00 54 283  compared to something like 2 days       Yes  that s correct   1 million nodes converted in less than a minute and 100 000 nodes in under 4 seconds   You can read about the new method and get a copy of the code at the following URL  http   www sqlservercentral com articles Hierarchy 94040   I also developed a  pre-aggregated  hierarchy using similar methods   MLM ers and people making bills of materials will be particularly interested in this article  http   www sqlservercentral com articles T-SQL 94570   If you do stop by to take a look at either article  jump into the  Join the discussion  link and let me know what you think

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This is really a square peg  round hole question   If relational databases and SQL are the only hammer you have or are willing to use  then the answers that have been posted thus far are adequate  However  why not use a tool designed to handle hierarchical data  Graph database are ideal for complex hierarchical data   The inefficiencies of the relational model along with the complexities of any code query solution to map a graph hierarchical model onto a relational model is just not worth the effort when compared to the ease with which a graph database solution can solve the same problem   Consider a Bill of Materials as a common hierarchical data structure   class Component extends Vertex       long assetId      long partNumber      long material      long amount      class PartOf extends Edge       class AdjacentTo extends Edge        Shortest path between two sub-assemblies  Simple graph traversal algorithm  Acceptable paths can be qualified based on criteria   Similarity  What is the degree of similarity between two assemblies  Perform a traversal on both sub-trees computing the intersection and union of the two sub-trees  The percent similar is the intersection divided by the union   Transitive Closure  Walk the sub-tree and sum up the field s  of interest  e g   How much aluminum is in a sub-assembly    Yes  you can solve the problem with SQL and a relational database  However  there are much better approaches if you are willing to use the right tool for the job

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Adjacency Model   Nested Sets Model  I went for it because I could insert new items to the tree easily  you just need a branch s id to insert a new item to it  and also query it quite fast    ------------- ---------------------- -------- ----- -----    category id   name                   parent   lft   rgt    ------------- ---------------------- -------- ----- -----              1   ELECTRONICS              NULL     1    20               2   TELEVISIONS                 1     2     9               3   TUBE                        2     3     4               4   LCD                         2     5     6               5   PLASMA                      2     7     8               6   PORTABLE ELECTRONICS        1    10    19               7   MP3 PLAYERS                 6    11    14               8   FLASH                       7    12    13               9   CD PLAYERS                  6    15    16              10   2 WAY RADIOS                6    17    18    ------------- ---------------------- -------- ----- -----     Every time you need all children of any parent you just query the parent column  If you needed all descendants of any parent you query for items which have their lft between lft and rgt of parent  If you needed all parents of any node up to the root of the tree  you query for items having lft lower than the node s lft and rgt bigger than the node s rgt and sort the by parent    I needed to make accessing and querying the tree faster than inserts  that s why I chose this  The only problem is to fix the left and right columns when inserting new items  well I created a stored procedure for it and called it every time I inserted a new item which was rare in my case but it is really fast  I got the idea from the Joe Celko s book  and the stored procedure and how I came up with it is explained here in DBA SE https   dba stackexchange com q 89051 41481

[sql] What are the options for storing hierarchical data in a relational database?

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