Alternate Data Model (Hierarchical, Relational and Network)

Hierarchical Model:

A hierarchical database consists of a collection of records which are connected to one another through links. A record is a collection of fields where each field contains only one data value. A link is an association between precisely two records. The schema for a hierarchical database consists of boxes which correspond to record types and lines which correspond to links. The record types are organized in the form of a rooted tree. There does not exist any cycles in the underlying graph. Relationships formed in the graph must be in such a way that only one-to-many or one-to-one relationships exist between a parent and a child. If a segment has no parent then it is called "root" and if a segment has no children then it is called "leaf". A parent may have an arrow pointing to a child, but a child must have an arrow pointing to its parent. The database schema is represented as a collection of tree-structure diagrams.

• a single instance of a database tree
• The root of this tree is a dummy node
• The children of that node are actual instances of the appropriate record type.

The particular data corresponding to the segment type is referred to as Segment instance. A Parent-Child relationship is 1:N relationship between two segments.

When we transform E-R diagrams into corresponding tree-structure diagrams, we must ensure that the resulting diagrams are in the form of rooted trees. For example, E-R diagrams with two entity sets, customer, and account, related through a binary, one-to-many relationship depositor. The record type customer has three fields: customer_name, customer_street, and customer_city.The record type account has two fields: account_number and balance. The link depositor points an arrow to the customer.

Relational Model

The Relational model was initially proposed by Dr. E.F. Codd of IBM in 1970 in the following paper as "A Relational Model for Large Shared Data Banks," Communications of the ACM , June 1970. This model is based on a concept of a Relation. A Relation is a mathematical concept based on the ideas of sets. The strength of the relational approach to data management comes from the formal foundation provided by the theory of relations. The relational model is most widely used data model for commercial data-processing. The reason behind this is its simplicity and easy maintenance. The model is based on a collection of tables. Users of the database can create tables, insert new tables or also modify the existing tables.

The relation is formed over the Cartesian product of the sets where each set has values from a domain which is used in a specific role which is conveyed by the attribute name. Formally, Given R(A1,A2,.....,An) where A1, A2,.....,An are attributes

R=( A1,A2,.....,An) is a relation schema

• r(R) denotes a relation r on the relation schema R
• For example: Customer_schema= (F_name, L_name, Mobile, District, VDC, Ward_Num)
• customer(Customer_schema)

The name of a relation and the set of attributes for a relation is called a Schema. The schema for the relation with the relation name followed by a parenthesized list of its attributes. The relational database schema is equal to the collection of relational schemas. Tuples are referred to as the rows of a relation except for the header row containing the attribute names. A tuple has one component for each attribute of the relation. A number of tuples present in the relation is called Cardinality of relation. Say, if there are four tuples in the relation, then the cardinality of this relation is 4. A number of attribute/fields present in the relation is called Degree/Arity relation. If there are four attributes in the relation, then the degree of this relation is 4.

Characteristics of Relation

• The ordering of Tuples in a relation r(R): The tuples are not considered to be ordered, even though they appear to be in the tabular form.
• Ordering the attributes in a relation schema R (and of values within each tuple): Consider the attributes in R(A1,A2,.....,An) and the values in t=<v1,v2,....,vn> to be ordered.
• Values in a tuple: All values are considered atomic i.e. indivisible. A special null value is used to represent values that are unknown or inapplicable to certain tuples.
• Notation: We refer component values of a tuple t by t[Ai] = vi ( the value of attribute Ai for tuple t). Similarly, t[Au, Av,....., Aw] refers to the sub-tuple of t containing the value of attributes Au, Av,...., Aw respectively.

Network Model

A network database is known as the extension of the hierarchical database. In this model also, data elements of a database are organized to have parent-child relationships, and all types of relationships among data elements must be determined when a database is first designed. In a network database, however, a child data element can have more than one parent element or no parent at all. Moreover, in this type of database, database management system permits extraction of needed information by beginning with any data element in database structure, instead of starting from root data element.

References:

1. H.F.Korth and A. Silberschatz,"Database system concepts",McGraw Hill,2010
2. A.K.Majumdar and p, Bhattacharya,"Database Management Systems",Tata McGraw Hill,India,2004
3. F.Korth, Henry. Database System Concepts. 6th edition.