Entity-Relationship Model and Diagrams (continued) Todd S. Bacastow IST 210 Organization of data

Database Design Conceptual design Logical design Physical design Entity-Relationship Model Logical design Logical Schema Physical design

Critical Success Factors in Database Design Work interactively with the users as much as possible Follow a methodology throughout the data modeling process Incorporate structural and integrity considerations into the data models Combine conceptualization, normalization, and validation techniques into the methodology

Conceptual Design Phase

ERD as a “Semantic Model” Semantics: Relationships between symbols and their meanings. A semantic model attempts to capture the meaning of user's information and provide a concise, high-level description of that information. The information is represented by logical associations (relations) between pairs of objects and by the classification of objects into categories (entities).

Entity, Attribute, Relationship Name Address Numb CLIENT PASSPORT Exp A client must have a passport Each client has only one passport A client may have one or more itinerates An itinerary belongs to one client ITINERARY Date city

Basic relationships One-to-One One-to-Many Many-to-Many HUSBAND WIFE A husband has one wife A wife has one husband One-to-Many TEAM PARTICIPANT A team has many participants Many participants belong to one team Many-to-Many EMPLOYEE SKILL An employee has many skills Many employees have the same skill

Exercise Business rules: A university where a student may take many courses employs lecturers. Each lecturer teaches one or more courses but no course is taught by more than one lecturer. Each student has to complete two or three assignments for each course. Requirement: Draw an entity relationship (ER) diagram for the situation described.

Solution

Review Use ER Diagrams to help understand The essential information Visual guide to the organization Plan for how to proceed Improve database design

Logical design phase Conceptual E-R Model 1. REFINE THE CONCEPTUAL MODEL Refined Conceptual Model 2.APPLY THE RULES OF NORMALIZATION Logical Data Model

The Next Step Going from this: To this: Table1 Table2 Table3 Table4

Logical Database Design Main objectives of relational databases is to ensure that each item of data is held only once within the database: Minimize the amount of storage space Simplify updating procedures Ensure that data is accurate Done by converting ERD into well-structured relational data model

Relational Data Model E-R Model helps to define the underlying tables CUSTOMER CUSTOMER receive INVOICE INVOICE

Converting ERD to tables Convert each of the entities (E1 & E2) in to a table. The attributes of the entity become attributes of the table. Convert each relationship (R1) with a many-to-many cardinality in to a table. The primary keys of the entities (E1 & E2) linked by the relationship and attributes of the relationship A1 become attributes of the resulting table. Do not convert relationships (R1) with one-to-many or one-to-one cardinality in to a table. For a one-to-one relationship, add the attributes of the relationship (A1) and the primary key of either table (T1 or T2) into the other table (T1 or T2). For a one-to-many relationship, add the attributes of the relationship (A1) and the primary key to the other table. Relationship1 R1 Entity 1 E1 Entity 2 E2 A1

Fields and Records Field: a single column of information in a table Appears as an attribute in the ERD Key: a field which uniquely identifies each record in a field Field Key

Fields and Records Foreign Key Foreign Key: links tables by referencing a key in another table Tuple: the full set of information about one occurrence of the entity All information included in a single row of the table Tuple

Review The key conversion tasks are: Create entity tables Resolve relationships Resolve multi-valued and composite attributes Insert primary and foreign keys Normalize to Third Normal Form (we will talk about this next time) Integrity constraints

Review Relations Just another name for a table Every relation has a unique name Every attribute value is atomic Every row is unique Attributes in tables have unique names The order of the columns is irrelevant The order of the rows is irrelevant