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through. For example, saying a phrase such as “The cat sat on the mat” would be hard to follow if you changed languages half way through. We will also see that some notations, like some languages, can express some ideas more clearly than others. 2. Relationships in an Entity-Relationship Diagram
Specifies maximum number of relationship instances in which each entity can participate. Types 1:1, 1:N, or M:N. Participation constraint. Specifies whether existence of entity depends on its being related to another entity. Types: total and partial.
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A comprehensive list of scientific root words, prefixes, and suffixes to help understand biological terminology.
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Jul 15, 2022 · A suffixes list can help you with reading, writing and vocabulary. The common suffix examples in this list will help you understand new words.
- Cheryl Cirelli
- B.A. English & Psychology
- admin@yourdictionary.com
- Contents
- What does Conceptual Design include?
- Entity Types, Entity Sets, Attributes and Keys
- Key attributes of an Entity Type
- Example of an Entity-Relationship Diagram
- Constraints on Relationship Types
- Design Choices for ER Conceptual Design
- Enhanced ER Modeling Concepts
- Subclasses, Superclasses, and Inheritance
- Specialization
- Generalization:
- Constraints on Specialization
- Translation of ER Schema into Tables
- Translating Relationship Types into Tables
- Summary of Conceptual Design
Basic concepts: entities and entity types, attributes and keys, relationships and relationship types Entity-Relationship schema (aka ER diagram) Constraints on relationship types Design choices Enhanced Entity-Relationship model features Steps in designing an ER schema Translation of an ER schema to tables
Ideas ! High-level ! design Relational database ! schema Relational DBMS Entity-Relationship model is used in the conceptual design of a database (+ conceptual level, conceptual schema) Design is independent of all physical considerations (DBMS, OS, . . . ). Questions that are addressed during conceptual design: { What are the entities and relati...
Entity: real-world object or thing with an independent existence and which is distinguishable from other objects. Examples are a person, car, customer, product, gene, book etc. Attributes: an entity is represented by a set of attributes (its descriptive properties), e.g., name, age, salary, price etc. Attribute values that describe each entity beco...
Entities of an entity type need to be distinguishable. superkey of an entity type is a set of one or more attributes whose values uniquely determine each entity in an entity set. candidate key of an entity type is a minimal (in terms of number of attributes) superkey. For an entity type, several candidate keys may exist. During conceptual design, o...
FName Account LName CAddress CUSTOMERS orders Quantity SUPPLIERS SName Prodname PRODUCTS offers Price Chain SAddress Customers-Suppliers-Products Entity-Relationship Diagram Rectangles represent entity types Ellipses represent attributes Diamonds represent relationship types Lines link attributes to entity types and entity types to relationship typ...
Limit the number of possible combinations of entities that may participate in a relationship set. There are two types of constraints: cardinality ratio and participation constraints Very useful concept in describing binary relationship types. For binary relationships, the cardinality ratio must be one of the following types:
It is possible to de ne entities and their relationships in a number of di erent ways (in the same model!). Should a real-world concept be modeled as an entity type, attribute, or relationship type? Is \Address" an attribute or an entity type? Decision depends upon the use one wants to make of address information. If one is interested in the struct...
Although most properties of entities and relationships can be expressed using the basic modeling constructs, some of them are costly and di cult to express (and to understand). That's why there are some extensions to the ER model.
In some cases, an entity type has numerous subgroupings of its entities that are meaningful and need to be represented explicitly because of their signi cance to the DB application. SSN PERSON Name ISA GPA STUDENT Major Relationships and attributes of superclass are inherited to subclass (in particular primary key attribute(s)); subclass can have a...
Process of de ning a set of subclasses of an entity type (top-down) SocialSN Name Address EMPLOYEES ISA Hours Wages Contractno HOURLY_EMPS CONTRACT_EMPS HOURLY EMPS is a subclass thus inherits its attributes and CONTRACT EMPS). of EMPLOYEES and relationships (same for
Reverse process of specialization (bottom-up); identify common features of entity types and generalize them into single superclass (including primary key!) VehicleNo Price LicensePlate VEHICLE ISA NoOfPassengers MaxSpeed Tonage Axels CAR TRUCK
disjointness and totality constraints disjoint, total/partial: each entity in the superclass must/can be in exactly one subclass The disjointness constraint is indicated by the word \disjoint" right next to the ISA triangle The totality constraint is indicated by double lines leading from the superclass to the ISA triangle overlapping constraints o...
An ER schema can be represented by a collection of tables which represent contents of the database (instance). Primary keys allow entity types and relationship types to be expressed uniformly as tables. For each entity and relationship type, a unique table can be derived which is assigned the name of the corresponding entity or relationship type. E...
A many-many relationship type is represented as a table with columns for the primary key attributes of the participating entity types, and any descriptive attributes of the relationship type. Example: Relationship type o ers Prodname and SName are the primary key attributes of the entity types SUPPLIERS and PRODUCTS. Translation of one-many and man...
Conceptual design follows requirements analysis, yields a high level description of data to be stored (conceptual level). ER model is a popular model for conceptual design, constructs are expressive, close to the way people think about applications; supported by many CASE tools. Basic constructs are entities, relationships, and attributes Some addi...
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The goal of conceptual design is to develop a set of data requirements that are comprehensive, clear & easy to understand, and algorithmically transformable. ER Diagrams (ERDs) are one such design model that visually represent the entities, attributes, and relationships of a system. Requirements elicitation and conceptual design is an iterative ...
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The Extended Entity-Relationship (EER) model is a conceptual. (or semantic) data model, capable of describing the data requirements for a new information system in a direct and easy to understand graphical notation. Data requirements for a database are described in terms of a. conceptual schema, using the EER model .
