Managing Hierarchical Data in a Relational Database

Who’s the old guy?  Robert Hatton  Data Architect at ARI Network Services  Building databases since the ‘70s  Author of ‘The Business Intelligence Team Handbook’  Feel free to contact me at

What is ‘Hierarchical Data’  Data elements related by Parent/Child and sibling relationships  Chart of Accounts  Bills of Material  Organization Charts  Catalogs  Outline style  Hierarchy is separate from data it organizes  Your reporting structure only helps folks navigate to your Employee record

The nature of hierarchies  Organized in levels  Number of levels  Width of levels  Single Parent vs. Multiple Parent  Different ID requirements

Hierarchy Terminology  Node  An individual member of a hierarchy  ROOT  The parent or ancestor of all other nodes  The Root has no parent  Leaf Node  A node without children  It’s a place to hang all of the data folks are interested in  Parent / Child / Sibling  They mean what they usually do

Node identification  String (or binary)  A single number won’t do!  Every level of hierarchy must be represented  Location in level has to be there too  Examples:  ‘/54/432/64/23/45’  ‘sporting goods, bicycles, mountain bikes, fat tire bikes, Fat Albert’

Example  Sporting Goods  1 Court sports  1.1 Tennis  1.2 Basketball  2 Field sports  2.1 Football  2.2 Baseball  3 Outdoor sports  3.1 Camping Gear  3.2 Bicycles  Road Bikes  Mountain Bikes  4 Drinking sports  4.1 Vocal Games  4.2 Skill Games  4.3 Card Games

Single Parent VS Multi-Parent Hierarchies  Know your data!  How you design your solution depends on data  Single Parent  If there is (or could be) a single node that could be in more than one branch, you can’t use this model!  Single parent model can house relationship key with the nodes.  Example: Chart of Accounts  Multi Parent  Relationship table has to be kept separate from nodes.  Example: Bills of Material.

 Bicycles  Bicycles.Felt  Bicycles.Felt.Mountain  Bicycles.Felt.Mountain.All Mountain  Bicycles.Felt.Mountain.All Mountain.Compulsion 10  Bicycles.Felt.Mountain.All Mountain.Compulsion 30  Bicycles.Felt.Mountain.Trail 27.5  Bicycles.Felt.Mountain.Trail 27.5.Decree 1  Bicycles.Felt.Mountain.Trail 27.5.Decree FRD  Bicycles.Felt.Road  Bicycles.Felt.Road.Aero  Bicycles.Felt.Road.Aero.AR FRD  Bicycles.Felt.Road.Aero.AR1 W  Bicycles.Felt.Road.Aero.AR2  Bicycles.Felt.Road.Race  Bicycles.LiteSpeed  Bicycles.LiteSpeed.Carbon  Bicycles.LiteSpeed.Titanium  Bicycles.LiteSpeed.Titanium.Mountain  Bicycles.LiteSpeed.Titanium.Mountain.All Mountain  Bicycles.LiteSpeed.Titanium.Mountain.All Mountain.Compulsion 10  Bicycles.LiteSpeed.Titanium.Mountain.All Mountain.Compulsion 30  Bicycles.LiteSpeed.Titanium.Mountain.Trail 27.5  Bicycles.LiteSpeed.Titanium.Mountain.Trail 27.5.Decree 1  Bicycles.LiteSpeed.Titanium.Mountain.Trail 27.5.Decree FRD Sample Hierarchy

Microsoft’s solution: HierarchyID data type  The hierarchyID data type is a variable length, system data type.  Example: /1/1/ or /1/2/ or /1/1.5  Root is represented by /  Supports ordering of siblings.  Intended to be housed in table holding hierarchy nodes.  Examples only show support for single parent hierarchies.  Supported in SQL Server 2008 and newer.

HierarchyID methods  GetAncestor  GetDescendant  GetLevel  GetRoot  IsDescendantOf  Parse  Read  GetReparentedValue  ToString  Write

Relationship table solution  Use a dedicated table to map relationships  Example  relationshipID  parentID  itemID  Key concept!  The parentID doesn’t point to the table housing node data.  It uses relationshipID from the mapping table  The itemID does point to the table housing node data.

Attribute tables  Item Attributes  Example fields  Item Description  Weight  QuantityOnHand  Price  Alternate tables  Employees  BillsOfMaterial  ChartOfAccounts  Relationship Attributes  Used by  Obsolete

Sample Schema

Demos  Children by Name  Children by ID  Ancestors by Name  Ancestors by ID  populateItemRelationshipPath

Thank you! 