The Art of SQL Server FILESTREAM - Red Gate Software

Chapter 11: FileTableIn addition to the predefined, fixed schema of FileTables, several additional databaseobjects are created when creating FileTables. These objects include DEFAULTconstraints to set appropriate values for many of the metadata columns in the FileTable,CHECK constraints to ensure that no invalid column values can be added (such as anon-NULL file_stream column if the row refers to a folder and not a file) and UNIQUEconstraints to ensure that each full path is unique. There is even a foreign key for theparent_path_locator column to ensure that it refers to a valid folder.Creating folder structures using the HIERARCHYIDdata typeFileTable supports folder structures with a maximum depth of 15 subfolders. Even thougha maximum of 15 subfolders is supported, you will probably create a maximum of 14subfolders, because it's not possible to create a file in the 15th level subfolder (as it wouldcreate a 16th level item).The folder structure is implemented using the HIERARCHYID feature introduced withSQL Server 2008. While a complete discussion of the HIERARCHYID data type is beyondthe scope of this book, it is valuable to review some of the concepts and basic queries thatcan be performed using HIERARCHYID fields.In a FileTable, the primary key is a field of type HIERARCHYID and even though it's easyto query the parent of a given HIERARCHYID value, the parent in the hierarchy is storedin the parent_path_locator field. To keep this discussion focused on the basics ofHIERARCHYID, and since we have yet to discuss how to create FileTables, we'll use asample table that does not actually contain any FILESTREAM data, but just the hierarchystructure and a name field, as shown in Listing 11-1.392