DBMS data models
- A database model is a collection of toos for describing data, data relationships, data semantic and consistency constraints.
- Underlying the structure of a database is the data model: a collection of conceptual tools for describing data, data relationships, data semantics, and consistency constraints.
- To illustrate the concept of a data model, we outline two data models in this section: the entity-relationship model and the relational model. Both provide a way to describe the design of a database at the logical level.
- They can be broadly classified into the following categories:
- Conceptual database models.
- Physical database models.
- Logical database models.
The Entity-Relationship Model
- The entity-relationship (E-R) data model is based on a perception of a real world that consists of a collection of basic objects, called entities, and of relationships among these objects.
- An entity is a “thing” or “object” in the real world that is distinguishable from other objects. For example, each person is an entity, and bank accounts can be considered as entities.
- A relationship is an association among several entities. For example, a depositor relationship associates a customer with each account that she has.
- The set of all entities of the same type and the set of all relationships of the same type are termed an entity set and relationship set, respectively.
- The overall logical structure (schema) of a database can be expressed graphically by an E-R diagram.
- The relational model uses a collection of tables to represent both data and the relationships among those data. Each table has multiple columns, and each column has a unique name.
- The data is arranged in a relation which is visually represented in a two dimensional table.
- The data is inserted into the table in the form of tuples (which are nothing but rows).
- A tuple is formed by one or more than one attributes, which are used as basic building blocks in the formation of various expressions that are used to derive a meaningful information.
- There can be any number of tuples in the table, but all the tuple contain fixed and same attributes with varying values.
- The relational model is implemented in database where a relation is represented by a table, a tuple is represented by a row, an attribute is represented by a column of the table, attribute name is the name of the column such as ‘identifier’, ‘name’, ‘city’ etc., attribute value contains the value for column in the row.
- The relational model represents the database as a collection of relations. Informally, each relation resembles a table of values or, to some extent, a flat file of records
- It is called a flat file because each record has a simple linear or flat structure.
- However, there are important differences between relations and files, as we shall soon see.
- When a relation is thought of as a table of values, each row in the table represents a collection of related data values