Introduction to HierarchyID in SQL Server

HierarchyID is a data type used in Microsoft SQL Server to represent hierarchical relationships between rows. It is part of the sys.types system view and provides methods for querying descendant relationships.

In this article, we will explore how to use HierarchyID to improve query performance and simplify complex relationships in your database.

Creating a Hierarchical Table Structure

To take advantage of HierarchyID, you need to add a new column called HierID to your table. This column will store the HierarchyID value for each row. The data type of this column is hierarchyid, which is a persisted data type that stores the actual hierarchy data.

Here’s an example of how to create the HierID column:

Alter Table Accounts add [HierID] as convert(hierarchyid,'/'+replace(AccountNumber,'-','/')+'/')  PERSISTED;

Querying Hierarchy

To query a range of accounts, you can use the HierID column. The between operator is used to specify the range.

Here’s an example of how to query all rows between two specific account numbers:

Declare @R1 varchar(50) = '1110-1-1'
Declare @R2 varchar(50) = '1120'

Select * 
  from Accounts
  Where HierID between convert(hierarchyid,'/'+replace(@R1,'-','/')+'/')
                   and convert(hierarchyid,'/'+replace(@R2+'-99999','-','/')+'/')

Querying Descendants

To query descendants of a specific account, you can use the IsDescendantOf method.

Here’s an example of how to query all descendants of a specific account:

Declare @S varchar(50) = '1110-1'

Select * 
  From Accounts
  Where HierID.IsDescendantOf( convert(hierarchyid,'/'+replace(@S,'-','/')+'/') ) = 1

Indexing for Performance

Creating an index on the HierID column can improve query performance. This is because the database can use the index to quickly locate rows that match a specific range.

Create Index IX_HierID ON Accounts (HierID);

Best Practices

• Use HierarchyID when you need to frequently query hierarchical relationships.
• Avoid using HierarchyID as a primary key column, as it is not unique by itself. Instead, use a separate column for the primary key.

By following these guidelines and techniques, you can improve your database’s performance and simplify complex relationships with HierarchyID.

Last modified on 2023-09-27