Hierarchical data is a common type of data that represents the relationships between entities in a tree-like structure. For example, you may have hierarchical data for your organizational structure, where each employee has a manager and a department; or for your file system, where each folder can contain subfolders and files; or for your website, where each page can have subpages and links.<\/p>\n\n\n\n
Storing and querying hierarchical data can be challenging in a relational database, where the data is organized in tables with rows and columns. You may need to use complex joins or recursive queries to traverse the hierarchy or find the ancestors or descendants of a given node. You may also need to use additional columns or tables to store the level or order of the nodes in the hierarchy.<\/p>\n\n\n\n
SQL Server (2008 and later) provides a solution for these challenges with the HierarchyID<\/strong> data type. The HierarchyID data type is a variable-length system data type that represents a position in a hierarchy. It has a special way of representing the relationships between the nodes in a hierarchy from top to bottom levels and from left to right among the children nodes of a parent node.<\/p>\n\n\n\n The HierarchyID data type uses a compact binary format that encodes the path from the root node to any node in the hierarchy. For example, the root node has a HierarchyID value of The HierarchyID data type also provides several methods that allow you to perform common hierarchical operations on HierarchyID values.<\/p>\n\n\n\n For example, you can use the Let’s say we have a table called Employees that stores the hierarchy of employees in an organization. <\/p>\n\n\n\n The table has the following columns:<\/p>\n\n\n\n The table has the following data:<\/p>\n\n\n Id<\/b><\/p>\n<\/td>\n Name<\/b><\/p>\n<\/td>\n Position<\/b><\/p>\n<\/td>\n Level<\/b><\/p>\n<\/td>\n<\/tr>\n 1<\/span><\/p>\n<\/td>\n Alice<\/span><\/p>\n<\/td>\n \/<\/span><\/p>\n<\/td>\n 0<\/span><\/p>\n<\/td>\n<\/tr>\n 2<\/span><\/p>\n<\/td>\n Bob<\/span><\/p>\n<\/td>\n \/1\/<\/span><\/p>\n<\/td>\n 1<\/span><\/p>\n<\/td>\n<\/tr>\n 3<\/span><\/p>\n<\/td>\n Carol<\/span><\/p>\n<\/td>\n \/2\/<\/span><\/p>\n<\/td>\n 1<\/span><\/p>\n<\/td>\n<\/tr>\n 4<\/span><\/p>\n<\/td>\n Dave<\/span><\/p>\n<\/td>\n \/1\/1\/<\/span><\/p>\n<\/td>\n 2<\/span><\/p>\n<\/td>\n<\/tr>\n 5<\/span><\/p>\n<\/td>\n Eve<\/span><\/p>\n<\/td>\n \/1\/2\/<\/span><\/p>\n<\/td>\n 2<\/span><\/p>\n<\/td>\n<\/tr>\n 6<\/span><\/p>\n<\/td>\n Frank<\/span><\/p>\n<\/td>\n \/2\/1\/<\/span><\/p>\n<\/td>\n 2<\/span><\/p>\n<\/td>\n<\/tr>\n 7<\/span><\/p>\n<\/td>\n Grace<\/span><\/p>\n<\/td>\n \/1\/1\/1\/<\/span><\/p>\n<\/td>\n 3<\/span><\/p>\n<\/td>\n<\/tr>\n 8<\/span><\/p>\n<\/td>\n Harry<\/span><\/p>\n<\/td>\n \/2\/1\/1\/<\/span><\/p>\n<\/td>\n 3<\/span><\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n The hierarchy looks like this:<\/span><\/p>\n Alice<\/span><\/p>\n \u251c\u2500\u2500 Bob<\/span><\/p>\n \u2502 \u00a0 \u251c\u2500\u2500 Dave<\/span><\/p>\n \u2502 \u00a0 \u2502 \u00a0 \u2514\u2500\u2500 Grace<\/span><\/p>\n \u2502 \u00a0 \u2514\u2500\u2500 Eve<\/span><\/p>\n \u2514\u2500\u2500 Carol<\/span><\/p>\n \u00a0\u00a0\u00a0\u00a0\u2514\u2500\u2500 Frank<\/span><\/p>\n \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u2514\u2500\u2500 Harry<\/span><\/p>\n\n\n The HierarchyID data type provides several methods that allow you to perform common hierarchical operations on HierarchyID values.<\/p>\n\n\n\n To find the parent node of a given node, you can use the GetAncestor()<\/strong> method with an argument of 1. For example, if you want to find the parent node of Eve, you can use GetAncestor (1) to return `\/1\/`, which is Bob’s position.\u00a0<\/p>\n\n\n\n The code would look like this:<\/p>\n\n\n\n To find the next child node of a given node, you can use the GetDescendant()<\/strong> method with two arguments that specify the left and right boundaries of the child node. For example, if you want to find the next child node of Bob, you can use GetDescendant (NULL, NULL) to return `\/1\/3\/`, which is a new position that does not exist yet, or GetDescendant (‘\/1\/2\/’, NULL) to return `\/1\/3\/`, which is also a new position. <\/p>\n\n\n\n The code would look like this:<\/p>\n\n\n\n To check if one node is a descendant of another node, you can use the IsDescendantOf()<\/strong> method with an argument that specifies the parent node. For example, if you want to check if Harry is a descendant of Alice, you can use IsDescendantOf (‘\/’) to return 1 (true), which means that Harry is a descendant of Alice; or IsDescendantOf (‘\/2\/’) to return 0 (false), which means that Harry is not a descendant of Carol. <\/p>\n\n\n\n The code would look like this:<\/p>\n\n\n\n Data that would benefit from the HierarchyID data type is more common than you might think. Here are some real-world examples of hierarchy structures that could potentially be in your SQL Server database:<\/p>\n\n\n\n You should use HierarchyID when\u2026<\/p>\n\n\n\n\/<\/code>, while its first child has a value of \/1\/<\/code>, and its second child has a value of \/2\/<\/code>. The first child of the first child has a value of \/1\/1\/<\/code>, and so on. This format allows you to compare and sort HierarchyID values easily using simple operators such as <<\/code>, ><\/code>, =, or <><\/code>.<\/p>\n\n\n\nGetAncestor()<\/code> method to find the parent node of a given node; or the GetDescendant()<\/code> method to find the next child node of a given node; or the IsDescendantOf()<\/code> method to check if one node is a descendant of another node.<\/p>\n\n\n\n\n
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\n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n Querying Hierarchical Data Using the HierarchyID Data Type<\/h2>\n\n\n\n
DECLARE @CurrentNode hierarchyid;\nSELECT @CurrentNode = Position FROM Employees WHERE Name = 'Eve';\nSELECT Name FROM Employees WHERE Position = @CurrentNode.GetAncestor(1); \n-- returns Bob<\/code><\/pre>\n\n\n\nDECLARE @CurrentNode hierarchyid;\nSELECT @CurrentNode = Position FROM Employees WHERE Name = 'Bob';\nSELECT @CurrentNode.GetDescendant(NULL, NULL); -- returns \/1\/3\/\nSELECT @CurrentNode.GetDescendant('\/1\/2\/', NULL); -- returns \/1\/3\/<\/code><\/pre>\n\n\n\nDECLARE @ChildNode hierarchyid;\nDECLARE @ParentNode hierarchyid;\nSELECT @ChildNode = Position FROM Employees WHERE Name = 'Harry';\nSELECT @ParentNode = '\/';\nSELECT @ChildNode.IsDescendantOf(@ParentNode); -- returns 1 (true)\nSELECT @ParentNode = '\/2\/';\nSELECT @ChildNode.IsDescendantOf(@ParentNode); -- returns 0 (false)<\/code><\/pre>\n\n\n\nExamples of \u201cHierarchical Data\u201d<\/h2>\n\n\n\n
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When Should I Use the HierarchyID Data Type?<\/h2>\n\n\n\n