International
Tables for
Crystallography
Volume G
Definition and exchange of crystallographic data
Edited by S. R. Hall and B. McMahon

International Tables for Crystallography (2006). Vol. G. ch. 2.6, p. 67

Section 2.6.6.1.8. ITEM_LINKED

J. D. Westbrook,a* H. M. Bermana and S. R. Hallb

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
Correspondence e-mail:  jwest@rcsb.rutgers.edu

2.6.6.1.8. ITEM_LINKED

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The ITEM_LINKED category defines parent–child relationships between data items. This provides the mechanism for specifying the relationships between data items that may exist in multiple categories. Link relationships are most commonly defined between key items, which form the keys for many different categories.

In the DDL definition, all child relationships are expressed within the parent category.

Because the item _item_linked.parent_name has been defined as an implicit item, the child relationships can be specified most economically in the parent category where the parent item name can be automatically inferred. If link relationships are specified in a child category, then both parent and child item names must be specified.

Both parent and child item names in this category are children of _item.name, which ensures that all link relationships can be properly resolved. However, it is possible to define cyclical link relationships within this category. Any implementation of this DDL category should include a method to check for the existence of such pathological cases.








































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