Download Chapter 9: Designing Databases.... Show
Modern Systems Analysis and Design Sixth Edition Jeffrey A. Hoffer Joey F. George Joseph S. Valacich Chapter 9 Designing Databases Learning Objectives Concisely define each of the following key database design terms: relation, primary key, normalization, functional dependency, foreign key, referential integrity, field, data type, null value, denormalization, file organization, index, and secondary key. Explain the role of designing databases in the analysis and design of an information system. Transform an entity-relationship (E-R) diagram into an equivalent set of well-structured (normalized) relations. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 2 Learning Objectives (Cont.) Merge normalized relations from separate user views into a consolidated set of well-structured relations. Choose storage formats for fields in database tables. Translate well-structured relations into efficient database tables. Explain when to use different types of file organizations to store computer files. Describe the purpose of indexes and the important considerations in selecting attributes to be indexed. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 3 Introduction FIGURE 9-1 Systems development life cycle with design phase highlighted Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 4 Database Design File and database design occurs in two steps. 1. Develop a logical database model, which describes data using notation that corresponds to a data organization used by a database management system. 2. Prescribe the technical specifications for computer files and databases in which to store the data. Relational database model Physical database design provides specifications Logical and physical database design in parallel with other system design steps Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 5 FIGURE 9-2 Relationship between data modeling and the systems development life cycle Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 6 The Process of Database Design (Cont.) Four key steps in logical database modeling and design: 1. 2. 3. 4. Chapter 9 Develop a logical data model for each known user interface for the application using normalization principles. Combine normalized data requirements from all user interfaces into one consolidated logical database model (view integration). Translate the conceptual E-R data model for the application into normalized data requirements. Compare the consolidated logical database design with the translated E-R model and produce one final logical database model for the application. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 7 Physical Database Design Key physical database design decisions include: Chapter 9 Choosing a storage format for each attribute from the logical database model. Grouping attributes from the logical database model into physical records. Arranging related records in secondary memory (hard disks and magnetic tapes) so that records can be stored, retrieved and updated rapidly. Selecting media and structures for storing data to make access more efficient. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 8 Deliverables and Outcomes Logical database design Must account for every data element on a system input or output. Normalized relations are the primary deliverable. Physical database design Converts Chapter 9 relations into database tables. Programmers and database analysts code the definitions of the database. Written in Structured Query Language (SQL). © 2011 Pearson Education, Inc. Publishing as Prentice Hall 9 FIGURE 9-3 (d) Conceptual data model and transformed relations Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 10 Relational Database Model Relational database model: data represented as a set of related tables or relations Relation: a named, two-dimensional table of data; each relation consists of a set of named columns and an arbitrary number of unnamed rows Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 11 Relational Database Model (Cont.) Relations have several properties that distinguish them from nonrelational tables: Entries in cells are simple. Entries in columns are from the same set of values. Each row is unique. The sequence of columns can be interchanged without changing the meaning or use of the relation. The rows may be interchanged or stored in any sequence. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 12 Well-Structured Relation and Primary Keys Well-Structured Relation (or table) Primary Key An attribute whose value is unique across all occurrences of a relation All relations have a primary key. Chapter 9 A relation that contains a minimum amount of redundancy Allows users to insert, modify, and delete the rows without errors or inconsistencies This is how rows are ensured to be unique. A primary key may involve a single attribute or be composed of multiple attributes. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 13 Normalization and Rules of Normalization Normalization: the process of converting complex data structures into simple, stable data structures The result of normalization is that every nonprimary key attribute depends upon the whole primary key. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 14 Normalization and Rules of Normalization (Cont.) First Normal From (1NF) Second Normal Form (2NF) Unique rows, no multivalued attributes All relations are in 1NF Each nonprimary key attribute is identified by the whole key (called full functional dependency) Third Normal Form (3NF) Nonprimary key attributes do not depend on each other (i.e. no transitive dependencies) 2/14/2015Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 15 Functional Dependencies and Primary Keys Functional Dependency: a particular relationship between two attributes For a given relation, attribute B is functionally dependent on attribute A if, for every valid value of A, that value of A uniquely determines the value of B. The functional dependence of B on A is represented by A→B. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 16 Functional Dependencies and Primary Keys (Cont.) Functional dependency is not a mathematical dependency. Instances (or sample data) in a relation do not prove the existence of a functional dependency. Knowledge of problem domain is most reliable method for identifying functional dependency. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 17 Second Normal Form (2NF) A relation is in second normal form (2NF) if any of the following conditions apply: The primary key consists of only one attribute. No nonprimary key attributes exist in the relation. Every nonprimary key attribute is functionally dependent on the full set of primary key attributes. To convert a relation into 2NF, you decompose the relation into new relations using the attributes, called determinants, that determine other attributes. The determinants are the primary key of the new relation. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 18 Third Normal Form (3NF) A relation is in third normal form (3NF) if it is in second normal form (2NF) and there are no functional (transitive) dependencies between two (or more) nonprimary key attributes. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 19 Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 20 Third Normal Form (3NF) (Cont.) Foreign Key: an attribute that appears as a nonprimary key attribute in one relation and as a primary key attribute (or part of a primary key) in another relation Referential Integrity: an integrity constraint specifying that the value (or existence) of an attribute in one relation depends on the value (or existence) of the same attribute in another relation Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 21 Transforming E-R Diagrams into Relations It is useful to transform the conceptual data model into a set of normalized relations. Steps Represent entities. Represent relationships. Normalize the relations. Merge the relations. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 22 Representing Entities Each regular entity is transformed into a relation. The identifier of the entity type becomes the primary key of the corresponding relation. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 23 Representing Entities The primary key must satisfy the following two conditions. The value of the key must uniquely identify every row in the relation. The key should be nonredundant. The entity type label is translated into a relation name. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 24 Binary 1:N and 1:1Relationships The procedure for representing relationships depends on both the degree of the relationship – unary, binary, ternary – and the cardinalities of the relationship. Binary 1:N Relationship is represented by adding the primary key attribute (or attributes) of the entity on the one side of the relationship as a foreign key in the relation that is on the many side of the relationship. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 25 Binary 1:N and 1:1Relationships (Cont.) Binary or Unary 1:1 Relationship is represented by any of the following choices: Add the primary key of A as a foreign key of B. Add the primary key of B as a foreign key of A. Both of the above. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 26 Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 27 Binary and Higher-Degree M:N Relationships Chapter 9 Create another relation and include primary keys of all relations as primary key of new relation © 2011 Pearson Education, Inc. Publishing as Prentice Hall 28 Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 29 Unary Relationships Unary 1:N Relationship Chapter 9 Is modeled as a relation Primary key of that relation is the same as for the entity type Foreign key is added to the relation that references the primary key values Recursive foreign key: a foreign key in a relation that references the primary key values of that same relation © 2011 Pearson Education, Inc. Publishing as Prentice Hall 30 Unary Relationships Unary M:N Relationship Chapter 9 Is modeled as one relation Create a separate relation the represent the M:N relationship Primary key of new relation is a composite key of two attributes that both take their values from the same primary key Any attribute associated with the relationship is included as a nonkey attribute in this new relation © 2011 Pearson Education, Inc. Publishing as Prentice Hall 31 FIGURE 9-13 Two unary relationships (a) EMPLOYEE with Manages relationship (1:N) (b) Bill-of-materials structure (M:N) Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 32 Merging Relations Purpose is to remove redundant relations The last step in logical database design Prior to physical file and database design Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 33 View Integration Problems Must understand the meaning of the data and be prepared to resolve any problems that arise in the process Synonyms: two different names used for the same attribute When merging, get agreement from users on a single, standard name Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 34 View Integration Problems (Cont.) Homonyms: a single attribute name that is used for two or more different attributes. Resolved by creating a new name Dependencies between nonkeys— dependencies may be created as a result of view integration To resolve, the new relation must be normalized Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 35 View Integration Problems (Cont.) Class/Subclass — relationships may be hidden in user views or relations Resolved Chapter 9 by creating a new name © 2011 Pearson Education, Inc. Publishing as Prentice Hall 36 FIGURE 9-16 Class diagram corresponding to normalized relations of Hoosier Burger‘s inventory control system Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 37 Relations for Hoosier Burger Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 38 Physical File and Database Design The following information is required: Normalized relations, including volume estimates Definitions of each attribute Descriptions of where and when data are used, entered, retrieved, deleted, and updated (including frequencies) Expectations or requirements for response time and data integrity Descriptions of the technologies used for implementing the files and database Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 39 Designing Fields (Cont.) Field: the smallest unit of named application data recognized by system software Attributes from relations will be represented as fields Data Type: a coding scheme recognized by system software for representing organizational data Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 40 Choosing Data Types Selecting a data type balances four objectives: Minimize storage space. Represent all possible values of the field. Improve data integrity of the field. Support all data manipulations desired on the field. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 41 Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 42 Calculated Fields Calculated (or computed or derived) field: a field that can be derived from other database fields It is common for an attribute to be mathematically related to other data. The calculate value is either stored or computed when it is requested. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 43 Controlling Data Integrity Default Value: a value a field will assume unless an explicit value is entered for that field Range Control: limits range of values that can be entered into field Chapter 9 Both numeric and alphanumeric data Referential Integrity: an integrity constraint specifying that the value (or existence) of an attribute in one relation depends on the value (or existence) of the same attribute in another relation Null Value: a special field value, distinct from zero, blank, or any other value, that indicates that the value for the field is missing or otherwise unknown © 2011 Pearson Education, Inc. Publishing as Prentice Hall 44 Designing Physical Tables Chapter 9 Relational database is a set of related tables. Physical Table: a named set of rows and columns that specifies the fields in each row of the table Denormalization: the process of splitting or combining normalized relations into physical tables based on affinity of use of rows and fields Denormalization optimizes certain data processing activities at the expense of others. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 45 Designing Physical Tables (Cont.) Three types of table partitioning: Range partitioning: partitions are defined by nonoverlapping ranges of values for a specified attribute Hash partitioning: a table row is assigned to a partition by an algorithm and then maps the specified attribute value to a partition Composite partitioning: combines range and hash partitioning by first segregating data by ranges on the designated attribute, and then within each of these partitions Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 46 Designing Physical Tables (Cont.) Various forms of denormalization, which involves combining data from several normalized tables, can be done. No hard-and-fast rules for deciding Three common situations where denormalization may be used: Chapter 9 Two entities with a one-to-one relationship A many-to-many relationship (associative entity) with nonkey attributes Reference data © 2011 Pearson Education, Inc. Publishing as Prentice Hall 47 File Organizations File organization: a technique for physically arranging the records of a file Physical file: a named set of table rows stored in a contiguous section of secondary memory Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 48 File Organizations (Cont.) Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 49 File Organizations (Cont.) Sequential file organization: a file organization in which rows in a file are stored in sequence according to a primary key value Hashed file organization: a file organization in which the address for each row is determined using an algorithm Pointer: a field of data that can be used to locate a related field or row of data Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 50 Arranging Table Rows Objectives for choosing file organization Chapter 9 Fast data retrieval High throughput for processing transactions Efficient use of storage space Protection from failures or data loss Minimizing need for reorganization Accommodating growth Security from unauthorized use © 2011 Pearson Education, Inc. Publishing as Prentice Hall 51 Indexed File Organization Indexed file organization: a file organization in which rows are stored either sequentially or nonsequentially, and an index is created that allows software to locate individual rows Index: a table used to determine the location of rows in a file that satisfy some condition Secondary keys: one or a combination of fields for which more than one row may have the same combination of values Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 52 Indexed File Organization (Cont.) Main disadvantages: Main advantage: Extra space required to store the indexes Extra time necessary to access and maintain indexes Allows for both random and sequential processing Guidelines for choosing indexes Chapter 9 Specify a unique index for the primary key of each table. Specify an index for foreign keys. Specify an index for nonkey fields that are referenced in qualification, sorting and grouping commands for the purpose of retrieving data. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 53 Designing Controls for Files Two of the goals of physical table design are protection from failure or data loss and security from unauthorized use. These goals are achieved primarily by implementing controls on each file. Two other important types of controls address file backup and security. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 54 Designing Controls for Files (Cont.) Techniques for file restoration include: Periodically making a backup copy of a file. Storing a copy of each change to a file in a transaction log or audit trail. Storing a copy of each row before or after it is changed. Means of building data security into a file include: Chapter 9 Coding, or encrypting, the data in the file. Requiring data file users to identify themselves by entering user names and passwords. Prohibiting users from directly manipulating any data in the file by forcing users to work with a copy (real or virtual). © 2011 Pearson Education, Inc. Publishing as Prentice Hall 55 Physical Database Design for Hoosier Burger The following decisions need to be made: Chapter 9 Decide to create one or more fields for each attribute and determine a data type for each field. For each field, decide if it is calculated; needs to be coded or compressed; must have a default value or picture; or must have range, referential integrity, or null value controls. For each relation, decide if it should be denormalized to achieve desired processing efficiencies. Choose a file organization for each physical file. Select suitable controls for each file and the database. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 56 Electronic Commerce Application: Designing Databases Designing databases for Pine Valley Furniture’s WebStore Review the conceptual model (E-R diagram). Examine the lists of attributes for each entity. Complete the database design. Share all design information with project team to be turned into a working database during implementation. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 57 Summary In this chapter you learned how to: Concisely define each of the following key database design terms: relation, primary key, normalization, functional dependency, foreign key, referential integrity, field, data type, null value, denormalization, file organization, index, and secondary key. Explain the role of designing databases in the analysis and design of an information system. Transform an entity-relationship (E-R) diagram into an equivalent set of well-structured (normalized) relations. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 58 Summary (Cont.) Merge normalized relations from separate user views into a consolidated set of well-structured relations. Choose storage formats for fields in database tables. Translate well-structured relations into efficient database tables. Explain when to use different types of file organizations to store computer files. Describe the purpose of indexes and the important considerations in selecting attributes to be indexed. Chapter 9 © 2011 Pearson Education, Inc. Publishing as Prentice Hall 59 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher. Printed in the United States of America. Copyright © 2011 Pearson Education, Inc. Publishing as Prentice Hall Which of these describes a purpose of logical model and physical database design?Which of the following is a purpose of logical and physical database design? The preparation of a final conceptual model and the implementation of the database.
Which entities are used to associate two or more entities in order to have a many to many relationships?Dependent entities
These entities have the following characteristics: Dependent entities are used to connect two kernels together. They are said to be existence dependent on two or more tables. Many to many relationships become associative tables with at least two foreign keys.
Which best describes a column in a relational database table?In the context of relational databases, a column is a set of data values, all of a single type, in a table. Columns define the data in a table, while rows populate data into the table. Most databases allow columns to contain complex data like images, whole documents or even video clips.
Which of the following defines its entities and the relationship among them?A database schema defines its entities and the relationship among them. It contains a descriptive detail of the database, which can be depicted by means of schema diagrams. It's the database designers who design the schema to help programmers understand the database and make it useful.
|
Combining all normalized user views into one consolidated logical database model refers to:
zusammenhängende Posts
Urheberrechte © © 2024 IlerngutCOM Inc.
