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International
Tables for Crystallography Volume G Definition and exchange of crystallographic data Edited by S. R. Hall and B. McMahon © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. G. ch. 2.6, p. 69
Section 2.6.6.2.5. CATEGORY_KEY
a
Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers, The State University of New Jersey, Department of Chemistry and Chemical Biology, 610 Taylor Road, Piscataway, NJ 08854-8087, USA, and bSchool of Biomedical and Chemical Sciences, University of Western Australia, Crawley, Perth, WA 6009, Australia |
The category CATEGORY_KEY identifies the data items within a category that form the basis for the category. The category basis uniquely identifies each group or tuple of items in the category. In the analogy of the category as a table, no row in a table may have duplicate values for its key data items.
The choice of basis has important consequences in the specification of a category. It is important to ensure that the key items that form the category basis can unambiguously identify any tuple of data items within the category. If this is not the case, then it may not be possible to reliably recover data items that are stored in the category. Because key items are required to address each tuple of items in a category, key items are considered mandatory items in the category.
It is interesting to note how the key data items have been selected for the categories that define the DDL, and how this choice of key items influences the structure of the DDL dictionary. In the DDL category CATEGORY_KEY, the basis includes both the identifier for the category, _category_key.id, and the name of the key data item, _category_key.name. This choice of basis allows for any unique groups of items in a category to be defined as key items. Duplicate key-item values within a category are forbidden by the data model. In the DDL category ITEM_TYPE, the basis includes only the identifier for the item name, _item_type.name. This choice of basis has the desired effect of limiting the specification of item data type, _item_type.code, to a single choice for each data item.
