This method of enforcing a relationship is preferred when code that isn't trusted to maintain the hierarchical tree has direct DML access to the table. ParentId AS EmployeeId.GetAncestor(1) PERSISTED To enforce a tree by using constraints, a computed column that defines the parent of each node can be created with a foreign key constraint back to the primary key ID. The above examples illustrate how an application can ensure that a tree is maintained. The following code populates the table with three rows and returns the results: INSERT Org_T2 (EmployeeId, EmployeeName) WHERE EmployeeId = Org_T2 SET LastChild = WHERE EmployeeId = Org_T2 (EmployeeId, SELECT = EmployeeId.GetDescendant(LastChild,NULL) SET TRANSACTION ISOLATION LEVEL SERIALIZABLE The following code computes the new node value within a serializable transaction: CREATE TABLE Org_T2ĬREATE PROCEDURE hierarchyid, nvarchar(50)) In addition to other error cases an application might want to check, a duplicate key violation after the insert indicates an attempt to add multiple employees with the same ID, and therefore must be recomputed. The Org_BreadthFirst index ensures that determining uses a range seek. Given two hierarchyid values a and b, a 0 GOTO INS_EMP This is rounded up to 40 bits, or 5 bytes, for storage. A node in an organizational hierarchy of 100,000 people with an average fanout of 6 levels takes about 38 bits. For small fanouts (0-7), the size is about 6*logA n bits, where A is the average fanout. The average number of bits that are required to represent a node in a tree with n nodes depends on the average fanout (the average number of children of a node). Values for hierarchyid have the following properties: Key Properties of hierarchyidĪ value of the hierarchyid data type represents a position in a tree hierarchy. Use the hierarchyid functions in Transact-SQL to query and manage hierarchical data. Use hierarchyid as a data type to create tables with a hierarchical structure, or to describe the hierarchical structure of data that is stored in another location.
Examples of the hierarchical data that is commonly stored in databases include the following:
Hierarchical relationships exist where one item of data is the parent of another item. Hierarchical data is defined as a set of data items that are related to each other by hierarchical relationships. hierarchyid is optimized for representing trees, which are the most common type of hierarchical data. The built-in hierarchyid data type makes it easier to store and query hierarchical data. Applies to: SQL Server (all supported versions) Azure SQL Database Azure SQL Managed Instance
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