Section 2.5.6.4. Relational attributes

This class of attributes is used to describe special relationships and dependencies between data items. These attributes are machine parsable and are formally defined in the DDL1 dictionary in Chapter 4.9 . They are

The attribute _category is used to specify to which group, or basis set, a data item belongs. The value of this attribute is a name string that identifies the group and it is usually the leading part of the tags for all items in this group. Data items in a list must have the same category value. Data items in the same category may, however, be divided into different lists, provided each list contains an appropriate key data item (see _list_reference below).

List-link attributes are used to specify dependencies between data items in different lists. The attribute _list_link_parent identifies an item in another list from which the defined item was derived. The parent data item, or items in the case of irreducible sets, must have a unique value within its own list. The _list_link_child attribute is declared in the definition of a parent item and identifies items in other lists that depend implicitly on the defined data item being present in the same data instantiation. The functionalities of these two attributes mirror each other. The parent item must be present in any data instantiation containing the child items, but not the converse. The definition examples in Figs 2.5.5.6 and 2.5.5.7 illustrate the use of these attributes.

The attributes _list_mandatory and _list_reference are also connected to each other. The former signals (with values of yes or no) whether the presence of a data item is essential to preserving the validity of a list of items. The latter attribute identifies the data item in the list that provides the unique key value to each packet or row of items in a list. A packet is made up of listed values, one for each item in the name list (i.e. the packet size matches the number of data names at the head of the list). The _list_reference attribute identifies items that are the keys to specific packets in the list. In Example 2 of Fig. 2.5.5.5 the key item is _atom_site_label and the listed labels S1, S2, N1 and C1 must be unique.

The attribute _list_uniqueness is used to identify items that must be unique for a list to be valid and accessible. This attribute is similar to _list_reference except that it appears only in a definition in which _list_mandatory is set to yes. This simplifies validation because it may be used as the placeholder for all items that jointly identify the uniqueness of a list packet. This is in contrast to the attribute _list_reference, which appears in the definition of every item dependent on this item.

Relational attributes are used to link equivalent data. The _related_item attribute identifies items related to the defined data item. The nature of this relationship is specified with the _related_function attribute according to the restricted value states of alternate, convention, conversion and replace. The definition of these states is detailed in Chapter 4.9 . Relational attributes are used to provide equivalent data items, to replace definitions when definitions are superseded or to change access pathways. These facilities are for archives, because they enable old data to be accessed and the associated definitions to remain in a dictionary even when superseded by new definitions. The old and new definitions are linked by these attributes so that all related data items can be validated and accessed.