The heirarchyid is a system data type available in SQL Server 2008 to represent hierarchies. It is based on a CLR data type, but is always available, whether the CLR is enabled or not.
The hierarchyid data type is a variable length, system data type. Use hierarchyid to represent position in a hierarchy. A column of type hierarchyid does not automatically represent a tree. It is up to the application to generate and assign hierarchyid values in such a way that the desired relationship between rows is reflected in the values.
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