Database Reference
In-Depth Information
A single-valued or derived attribute is transformed into a single field.
For a composite attribute, as many fields are used in the record type as the
number of component attributes in the composite attribute.
Instance Identifiers
The attribute or attributes forming the instance identifier maps into a set of
sequence fields.
Segment occurrences are sorted based on the values of the sequence fields.
A sequence field may be declared to be “unique” in the sense that no dupli-
cate values are allowed in the sequence field. This is similar to the function of
a primary key.
Only when a sequence field is defined to be “unique” it has a function similar
to that of a primary key.
Sequence fields may also be declared to be
“non-unique.”
Relationships Each relationship in the hierarchical data model becomes a binary
link between the parent and the child segment types.
One-to-One or One-to-Many Relationship
For each object set in the semantic data model, create a segment type.
If two object sets A and B are in a one-to-one or one-to-many relationship,
create a tree structure with A as the parent segment type and B as the child
segment type with link between A and B.
The object set on the “one” side of the relationship becomes the parent segment
type and the object set on the “many” side of the relationship becomes the child
segment type.
Many-to-Many Relationship
For each object set in the semantic data model, create a segment type.
If two object sets A and B are in a many-to-many relationship, map this as two
one-to-many relationships.
Create a tree structure with A as the parent segment type and B as the child
segment type with link between A and B.
Create another tree structure with B as the parent segment type and A as the
child segment type with link between B and A.
NETWORK DATA MODEL
The Conference on Data Systems Languages (CODASYL), the organization com-
prising of vendor representatives and user groups, developed the language COBOL.
In the late 1960s, CODASYL appointed a subgroup known as the Database Task
Group (DBTG) to develop standards for database systems. DBTG published a pre-
liminary report in 1969. Based on revisions and suggestions made for improvement,
DBTG published a revised version of the report in 1971.
Search WWH ::




Custom Search