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Hierarchical Database Management Systems


HIERARCHICAL – Database Management Systems

  1. HDMS is a tree like structure made up of nodes and branches
  2. In HDMS : A node is a collection of data attributes describing the entity (logically identifiable as a separate group) at that point
  3. In HDMS : The topmost node in the structure is called the root Nodes at succeeding lower levels are called children A hierarchical structure should satisfy these conditions:
  4. In HDMS : The data model should always start with a root node
  5. In HDMS : Every node consists of one or more attributes (fields) describing the entity (record) at that node
  6. In HDMS : Dependent nodes have to follow in the succeeding levels ; The node in the preceding level becomes the parent node of the new dependent (children) nodes  
  7. In HDMS : Every node occurring at level 2 has to be connected with one and only one node occurring at level 1
  8. In HDMS : A parent node can have one or more dependent child nodes (i.e, relationship between parent and child is One-to-Many)  once under each parent node
  9. In HDMS : Every node except, of course, the root has to be accessed
  10. In HDMS : If a child has more than one parent, it has to be duplicated  through its parent node
  11. In HDMS : There can be a number of occurrences of each node at each level

 

Let us consider an example as below :

Consider the two entities CUSTOMER-NA ME and LEDGER-NO. CUSTOMER-NAME has details of ADDRESS and PHONE. LEDGER-NO has details of ACCOUNT-NO and BALANCE.

The equivalent structure (with BANK as the root and CUSTOMER-NAME and LEDGER-NO as two parents):

The relationship is accomplished, by making available, the unique identifier (parent node) of one entity, in the relationship part of the unique identifier (parent node) of the other entity. The relationship is represented as:

More on HDMS

A child cannot be inserted without a parent node

The contents of a record do get duplicated in some other record, in the process of establishing a relationship

A child can be deleted without deleting its parent node

The root does not participate as a child

Now is the time to understand some of the pros and cons of using an HDMS

Pros :

  1. A relatively simple structure that's easy to use Familiar among data processing users Reduced data dependency Simplified (redefined) relationships
  2. A node that doesn't participate as a parent is called a leaf

Cons:

  1. Many-to-Many relationship can be implemented only in a clumsy way resulting in redundancy and inconsistency of stored data
  2. Deletion and Insertion operations become very complex Deletion of parent results in the deletion of all its children Any child is accessible only through its parent node Wastage of storage space is unavoidable

 


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