Discuss the correspondences between the ER model constructs …
Discuss the correspondences between the ER model constructs and the relational model constructs. Show how each ER model construct can be mapped to the relational model, and discuss any alternative mappings? List the three main approaches to database programming. What are the advantages and disadvantages of each approach?
Answer
The entity-relationship (ER) model and the relational model are two important conceptual models used in database design. The ER model is used to represent the entities and their relationships in a database, while the relational model represents data as tables with rows and columns. In this paper, we will explore the correspondences between the constructs of these two models and discuss how each ER model construct can be mapped to the relational model. We will also examine any alternative mappings available.
First, let us consider the basic constructs of the ER model and their corresponding constructs in the relational model. In the ER model, we have entities, attributes, and relationships. An entity represents a real-world object or concept, and is equivalent to a table in the relational model. The attributes of an entity correspond to the columns of a table, where each attribute value represents a specific property of the entity. Relationships in the ER model represent associations between entities, and can be represented in the relational model by using foreign keys to establish connections between tables.
To illustrate this correspondence, let us consider an example. Suppose we have an ER model with entities such as “Employee” and “Department”, where an employee belongs to a department. The “Employee” entity may have attributes like “ID”, “Name”, and “Salary”, while the “Department” entity may have attributes like “ID” and “Name”. In the relational model, we can create two tables, one for “Employee” and one for “Department”. The “Employee” table would have columns for “ID”, “Name”, “Salary”, and a foreign key column for the department ID. The “Department” table would have columns for “ID” and “Name”.
In addition to the basic constructs, the ER model also includes constructs like weak entities, cardinality/degree constraints, and inheritance. Weak entities, which depend on a strong entity for their existence, can be represented in the relational model by using foreign keys and table relationships. Cardinality/degree constraints, which describe the number of possible relationships between entities, can be represented by adding appropriate constraints to the table relationships. Inheritance, which allows entities to inherit attributes and relationships from other entities, can be represented using tables with specific relationships and foreign keys.
Alternative mappings may exist between the ER and relational models, depending on the specific requirements of a database. For example, inheritance in the ER model can be mapped to the relational model using separate tables for each entity type or using a single table with additional attributes to differentiate between entity types. The choice of mapping depends on factors such as the complexity of the hierarchy, the number of attributes inherited, and the frequency of querying.
Moving on to the three main approaches to database programming, we have procedural, relational, and object-oriented approaches. Procedural programming involves using programming languages to directly manipulate the database through procedures and functions. Relational programming, on the other hand, involves using SQL (Structured Query Language) to interact with the database through declarative queries. Object-oriented programming integrates database functionality into object-oriented languages, allowing for the storage and retrieval of objects in a transparent manner.
Each approach has its advantages and disadvantages. Procedural programming provides flexibility and control over the database operations, but can be prone to errors and difficulties in maintaining code. Relational programming offers a standardized and efficient way of querying data, but may lack flexibility for complex operations and may require expertise in SQL. Object-oriented programming provides a seamless integration of database and programming languages, but can be complex to implement and may not support all the features of a relational database management system.
In conclusion, the ER model constructs can be mapped to the relational model by representing entities as tables, attributes as columns, and relationships as foreign keys. Alternative mappings may exist for constructs like weak entities, cardinality/degree constraints, and inheritance. The three main approaches to database programming are procedural, relational, and object-oriented, each with their advantages and disadvantages. The selection of the appropriate approach depends on the specific requirements of the database and the skills of the developers.