Databases Reference
In-Depth Information
FOR EXAMPLE
Hierarchical Storage in the Real World
Though it isn't based on the hierarchical database model, hierarchical stor-
age has seen rapidly growing interest recently with the use of extensible
markup language (XML) files. XML files are commonly used for local data
storage and for data transfer over the Internet. XML is not a database type,
but is instead a data storage and presentation format. XML is used as an inter-
nal format, or at least a formatting option, in several database systems. How-
ever, some new database systems, designed primarily for small, low-volume
databases, are based on XML data files and storage structures. Even though
they use hierarchical storage, they are not hierarchical model databases.
but a parent can have multiple children. You might want to separate orders into
phone orders and mail orders. In that case, CUSTOMER may have PHONE
ORDER and MAIL ORDER as two child segments. The parent/child links are
maintained through
physical data pointers
that are embedded in the data
records. The parent has a pointer to the child record and the child has a pointer
back to its parent.
2.2.2 The Network Database Model
The network database model overcomes some of the limitations of the hierar-
chical database model and is more representative of real-world information
requirements. The network database model can represent most business infor-
mation. As you can see in Figure 2-5, the model doesn't attempt to force the
data into hierarchical levels. Notice the six data elements of sales territory, sales-
person, customer, order, order line item, and payment as nodes in a network
arrangement.
The
network database model
expresses relationships between two record
types by designating one as the owner record type and the other as the mem-
ber record type. For each owner, there are one or more member types. The owner
record type works somewhat like a hierarchical model parent and the member
record type like a hierarchical child. Each member type and its corresponding
owner record type form a set, which represents the relationship. A key differ-
ence between this and the hierarchical model is that each child (member) can
have multiple parents (owners). The relationships between the owner and mem-
ber are maintained by physical pointers in the data records. The owner points
to its members and each member points to its owner or owners.
