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In this section we describe some basic models. These are
following:-
The Hierarchical Model
A hierarchical database is a kind of database
management system that links records together in a tree data
structure such that each record type has only one owner, e.g. an
order is owned by only one customer. Hierarchical structures
were widely used in the first mainframe database management
systems. However, due to their restrictions, they often cannot
be used to relate structures that exist in the real world.
The original
concept for this model represents the data as a
hierarchical set of Parent/Child relations. Data
is basically organized in a tree of one or more
groups of fields, that are called segments.
Segments make up every single node of the tree.
Each child segment can only be related to just
one parent segment and access to the child
segment could only happen via its parent segment.
This means, that 1:n relations result in data
redundancy. To solve this problem, the data is
stored in only one place and is referenced
through links or physical pointers. When a user
accesses data he starts at the root level and
works down his way through the tree to the
desired target data. That's the reason why a user
must be very familiar with the data structure of
the whole database. But once he knows the
structure, data retrieval could become very fast.
Another advantage is built-in referential
integrity, which is automatically enforced.
However, because links between the segments are
hard-coded into the database, this model becomes
inflexible to changes in the data structure.
Hierarchical relationships between different types of data
can make it very easy to answer some questions, but very
difficult to answer others. If one-to-many relationship is
violated (e.g., a patient can have more than one physician) then
the hierarchy becomes a network.
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Similar to the network model.
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Organization of the records is as a collection of trees,
rather than arbitrary graphs.
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Figure shows a sample hierarchical database that is the
equivalent of the relational database
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Each department has several divisions with attributes of name
of manager, number of staffs, annual products etc.
Then each division has several sections with attributes such
as name of head, number of staff, number of PCs etc. Hierarchical model is a type of tree structure as shown
figure. A set of links connect all record types in a tree
structure.
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Advantages of hierarchical model
The advantages of hierarchical model are high speed of access
to large datasets and eases of updating. However the
disadvantage is that linkages are only possible vertically but
not horizontally or diagonally, that means there is no relation
between different trees at the same level unless they share the
same parent.
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