In this session, you will learn to: Map an ER diagram to a table Objectives

Reflects entities and their relationships, based on the data processing needs of an organization Can be mapped to a relational, hierarchical, or network model Is independent of individual applications, database management systems, hardware, and physical storage of data Conceptual Model

A database that conforms to an ER diagram can be represented by a collection of tables in the relational system. The mapping of ER diagrams to tables can be discussed in relation to the following: Regular entities Attributes Relationships Weak entities Subtypes and Supertypes Mapping ERDs to Tables

They can exist in isolation, independent of any other entity. They are the “building blocks” of the database. Each regular entity maps to a table. Regular Entities

Each property or attribute shown in the ER diagram maps to a column in the appropriate table. Attributes

Each type of relationship maps to tables in a different manner in the relational database management system. The most important principle is to create tables where information from the real world is stored and retrieved in an optimal way; that is, a minimum number of tables with a minimum number of attributes. In a relational system, a join operation retrieves all information by combining two or more tables. Are of the following types: One-to-One One-to-Many Many-to-Many Relationships

A weak entity is an entity whose existence depends on some other entity. It cannot exist if the entity on which it depends does not exist. Weak Entities

Each entity type (subtypes and supertypes) maps to a separate table. The primary key of the supertype is the foreign key of the subtype: It creates a link between the two. The foreign key of the subtype is also its primary key. Subtypes and Supertypes

Attributes: Do not introduce any unnecessary attributes. An attribute serves three purposes: To identify its owner entity To refer to another entity To simplify the description of an entity Keys: Relational systems require keys that uniquely identify the rows of a table. There are various types of keys: Primary Foreign Candidate Alternate Composite Tips on Logical Database Design

Any attribute (or set of attributes) that uniquely identifies a row in a table is a candidate for the primary key. Such an attribute is called a candidate key. Any attribute that is a candidate for the primary key but is not the primary key is called the alternate key. Keys can be simple or composite: A simple key is composed of a single attribute. A composite key, on the other hand, comprises two or more attributes. Tips on Logical Database Design (Contd.)

Just a minute Define the following terms: 1.Candidate Key 2.Alternate Key Answer: 1.Any attribute (or set of attributes) that uniquely identifies a row in a table is a candidate for the primary key. Such an attribute is called a candidate key. 2.Any attribute that is a candidate for the primary key but is not the primary key is called the alternate key.

Tips on Logical Database Design (Contd.) Entities: Some attributes may acquire further attributes during database design and become entities. Subentities: Replace optional attributes of an entity with subentities. This is called specialization. Specialization is the result of taking a subset of a higher-level entity set to form a lower-level entity set. Generalization is the result of taking the union of two or more lower-level entity sets to produce a higher ‑ level entity set.

Problem Statement Map the ER diagram to its corresponding tables. Demo: Mapping the ER Diagram to Tables

In this session, you learned that: The conceptual model reflects entities and their relationships. Data analysis helps determine entities and relationships. The conceptual model is independent of the system where it is to be implemented. Regular entities are not dependent. They can exist in isolation, independent of any other entity. Each entity maps to a table. Each attribute in an E/R diagram maps to an attribute in a table. Entities with common attributes should be merged. Attributes may acquire further attributes and become entities. The mapping of relationships depends on the type of relationship. Each type of relationship maps to tables in a different manner in the relational database management system. Summary

In one-to-one relationships, one instance of an entity can relate to only one instance of the related entity. In a one-to-many relationship, one instance of an entity can relate to more than one instance of the related entity. Many-to-many relationships map to tables. One-to-one relationships are not very common and may map to foreign keys in tables. A weak entity is an entity whose existence depends on some other entity. A subtype is a subset of another entity. A subtype is always dependent on supertype for its existence. The primary key of the supertype is the foreign key of the subtype. It creates a link between the two. Summary (Contd.)

A candidate key is a candidate for the primary key. An alternate key is a candidate key that is not a primary key. Optional attributes should be replaced with subentities; this is also called specialization. To simplify multiple references, a new superentity should be introduced; this is also called generalization. Summary (Contd.)