Relationship Types in Software

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Relationship Types
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In a relation mode, several types of relationships between the tables are possible. Hierarchical relationship type. In a hierarchical relation, an entity type (table) depends on the existence of previous (initial) entity types. The key of the initial entity type is inherited by the dependent entity type. Following the university example, the courses table (UKURS) is a dependent table on the faculties table (UFACH). The primary key from table UFACH is the faculty number (field FABNR) while the primary key from table UKURS is the combination of fields FABNR and KRSNR (course name). Aggregating relationship type. In an aggregating relationship, an entity type is produced by grouping two or more initial entities. The key of an aggregate table type is the combination of the keys from initial (previous or dependent) tables. In the university example included in R/3, the table of course 260
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Dependency Factors and Foreign Keys results and participation, table UKRTB, is an aggregated type of table which is created using tables UFACH (faculty table), UKURS (courses table), and UPERS (people at university). From each of these tables, the new aggregated table, UKRTB, inherits its primary key: FABNR (faculty number), SEMNR (seminar number), and IMMNR (matriculation number). Referential relationship type. In a referential relationship, an entity type always refers to the initial entity type but is not identified by it. The key of the referenced entity types is not affected by this. Conditional referential relationship type. This is similar to a referential relationship. However, the relationship link and therefore the referential aspect is application dependent (conditionally) for the entity type: the relationship might not always exist. Figure 8 3 shows an example of this type of relationship.
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Figure 8 3: Example of conditional referential relationship. Specialization. Specialization is a property of an entity type. Such an entity type represents a subset of one initial entity type, with the possibility of having additional attributes. The specializing entity type inherits the key of the initial entity type as a key attribute. Figure 8 4 shows an example where tables USTUD and UPROF are specializations of table UPERS.
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Figure 8 4: Example of specialization relationship. The relationship, as represented in the figure, means that there is one and only one element in the UPERS table for each element (record) from the USTUD table. One element from the UPERS table may or may not be referenced in table USTUD; but if it's there, it will be there only once. In other words, if all elements from the UPERS table were students, tables UPERS and USTUD are equivalent in the relationship, and then the cardinality would be 1:1.
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Dependency Factors and Foreign Keys
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The dependency factor indicates the degree to which a table depends on another check table. It specifies whether a foreign key field should have value and whether this value can be modified without removing the unique identity of the record. The dependency factor defines the dependency levels from the point of view of the foreign key table. In SAP R/3, the dependency factor concept is implemented using the foreign key field type. Using foreign key 261
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Relational Operations field type, you can specify whether the foreign key fields are identifying or partially identifying the foreign key table, that is, whether they contain key fields or key candidates. The following is copied from SAP online documentation. (Copyright by SAP AG.) Possible foreign key types are: Non key fields / candidates. The foreign key fields are neither primary key fields of the check table nor do they uniquely identify each record of the check table (key candidates). The foreign key fields are thus not (partially) identifying for the foreign key table. Key fields / candidates. The foreign key fields are either primary key fields of the check table or else they uniquely identify each record of the check table (key candidates). The foreign key fields are thus (partially) identifying for the foreign key table. Key fields of a text table. The foreign key table is a text table for the check table. That is, with the exception of the language key field, the key of this table is the same as that of the check table. This is a special instance of the key fields/candidates type. Entry of the semantic attributes for a foreign key relationship is optional. For this reason, the foreign key field type also provides the option not specified.
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