data models n.
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  1. DATA MODELS • A collection of conceptual tools for describing data, data relationships, data semantics, and consistency constraints. • Provide a way to describe the design of a database at the logical level. • Three data models that are used for database management are:  1. The Relational Data Model • 2. The Hierarchical Data Model • 3. The Network Data Model

  2. The Relational Data Model • In this model,the data is organized into tables (ie., rows and columns). These tables are called relations. • A row in a table represents a relationship among a set of values. • Consider a sample database with three tables (relations):Suppliers, Items, Quantities: • Suppliers (sup-no#,sup-name,status, city) • Items ( item-no#, item-name, price) • Quantities (sup-no#, item-no#, Qty)

  3. Suppliers Items Quantities

  4. Rows of relations are generally referred to as tuplesand the columns are usually referred to as attributes. • No two tuples are identical and their ordering is not important. • A relationship is represented by combining the primary key of the relations. • various operations can be performed on relations such as insert new tuples, delete tuples and modify tuples. • There are several languages for expressing these operations. One such language is relational query language.

  5. The Network Data Model • In this model, data is represented by collections of recordsand relationships among data are represented by links. Ie., the collection of records which are connected to one another by means of links. • A record is a collection of fields (attributes), each of which contains only one data value. • A link is an association between precisely two records. • The structure of the database is shown as an arbitrary graph in which the records form the nodes and links form the edges. • There is no concept of a root node.

  6. 10 20 20 20 10 10 Consider the structure of the above sample database of suppliers, items and quantities.

  7. The operations on a network database are performed through a data manipulation language for network model. • The operations that can be performed on a network database include find, insert, delete, modify etc. • The inserting or removing records include connect, disconnect and reconnect operations.

  8. The Hierarchical Data Model • Same as network model. The only difference is that in the hierarchical model, records are organized as trees rather than arbitrary graphs. • The record type at the top of the tree is usually known as root. • In general, the root may have any number of dependents and each of these dependents may have any number of low level dependents and so on. • Consider the hierarchical view of the above sample database of suppliers, items and quantities.

  9. 10 20 20 20 10 10

  10. In this tree structure supplier record type is the root node. • Here the relationship is expressed in the form of a tree with all the edges pointing to the leaf. • The operations on a hierarchical database are performed through a data manipulation language for hierarchical data model. • The various operations include retrieval, insertions , deletions and modifications of records.