RDBMS
A data model is a collection of high-level data description constructs that hide many low-level storage details A semantic data model is a more abstract, high-level data model that makes it easier for a user to come up with a good initial description of the data in an enterprise. A database design in terms of a semantic model serves as a useful starting point and is subsequently translated into a database design in terms of the data model the DBMS actually supports. A widely used semantic data model called the entity-relationship (ER) model allows us to pictorially denote entities and the relationships among them
1.5.1 The Relational Model
The central data description construct in this model is relation, which can be thought of as a set of records. A description of data in terms of a data model is called a schema. The schema for a relation specifies its name, the name of each field or attribute or column. Example: student information in a university database my be stored in a relation with the following schema (with 5 fields): Students(sid: string, name: string, login: string, age: integer, gpa: real) An example instance of the Students relation: sid name login age gpa 53666 Jones jones@cs 18 3.4 53588 Smith smith@ee 18 3.2 Each row in the Students relation is a record that describes a student. Every row follows the schema of the Student relation and schema can therefore be regarded as a template for describing a student. We can make the description of a collection of students more precise by specifying integrity constraints, which are conditions that the records in a relation must staisfy. Other notable models: hierarchial model, network model, object-oriented model, and the object-relational model.
1.5.2 Levels of Abstraction in a DBMS
A data definition language (DDL) is used to define the external and conceptual schemas.
1.5.1 The Relational Model
The central data description construct in this model is relation, which can be thought of as a set of records. A description of data in terms of a data model is called a schema. The schema for a relation specifies its name, the name of each field or attribute or column. Example: student information in a university database my be stored in a relation with the following schema (with 5 fields): Students(sid: string, name: string, login: string, age: integer, gpa: real) An example instance of the Students relation: sid name login age gpa 53666 Jones jones@cs 18 3.4 53588 Smith smith@ee 18 3.2 Each row in the Students relation is a record that describes a student. Every row follows the schema of the Student relation and schema can therefore be regarded as a template for describing a student. We can make the description of a collection of students more precise by specifying integrity constraints, which are conditions that the records in a relation must staisfy. Other notable models: hierarchial model, network model, object-oriented model, and the object-relational model.
1.5.2 Levels of Abstraction in a DBMS
A data definition language (DDL) is used to define the external and conceptual schemas.