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Database Schema CSC4480: Principles of Database Systems Lecture 2: Data Models/Entity Relationship Model Steps in Designing a Relational Database • Requirements and Specification – This involves scoping out the requirements and limitations of the database • Conceptual Design – This involves using a data model (e.g Entity Relationship Model) to represent the requirements of the specifications • Logical Design – This involves translating the conceptual design into tables and relationships expressed in a data model and implemented in a DBMS. Slide 2 Importance of Database Design • A foundation for good applications • Is a communication tool that facilitates the interaction among the designer, the application’s programmer, and the end user • Both an art and a science Slide 3 Defining Data Models • Models are simplified abstractions of real world events or conditions • A data model is the relatively simple representation using graphic, of facts (data) that are used to describe real world events, objects, or ideas – A data model illustrates what data are being represented (of interest for the end use or system) and how the data elements relate to each other – A data model represents data characteristics, relationships, constraints, and transformations. Slide 4 The importance of Data Models • Facilitates communication • Gives various views of the database • Organizes data for various users • Provides an abstraction for the creation of good a database Slide 5 Database Models • A database model is a collection of logical constructs (based on a data model) used to represent the data structure and the data relationships found within a specific database. • They include a set of concepts to describe the structure of a database, the operations for manipulating these structures, and certain constraints that the database should obey. Slide 6 Data Model Structure • Entity: Represents an object of interest to a user. Each entity describes an object in the real world. • Attributes: Characteristics that describes an entity • Relationships: Association between entities – One to Many (1:M) •One team consists of many players – Many to Many (M:N) •An employee might learn many job skills and each job skills might be learned by many employees •EMPLOYEE(M) learns SKILLS(N) – One to One (1:1) •A president leads one country Slide 7 • Identify the Model Characteristics for the database below: Slide 8 Data Model Structure (cont’d) • Constraints: A restriction placed on the data – E.g Salary must be greater than 0 and less than 350 – A GPA must be greater than or equal to 0.00 and less than or equal to 4.00 using 2 decimal places – A pilot cannot fly more than 10 hours during any 24 hour period. Slide 9 Naming Conventions • Entity name requirements – Be descriptive of the objects in the business environment – Use terminology that is familiar to the users • Attribute name – Required to be descriptive of the data represented by the attribute • Proper naming – Facilitates communication between parties – Promotes self-documentation Slide 10 In Class Exercise • What entities does uber have? • What attributes do you think Uber would have with the Entities you identified? • Describe relationships that might exist between each entities Slide 11 Business Rules • A business rule is a brief, precise and unambiguous description of a policy procedure or principle within an organization • Business rule are essential for: – Setting standards within an organization – Serving as a communication tool among users and designers – Describing the nature, role and scope of the data – Understanding of business processes – Developing the data model Slide 12 Business Rule Example • A Customer may generate many invoices • An invoice is generated by one customer Slide 13 Business Rule Example • A Customer may generate many invoices • An invoice is generated by one customer • What are some business rules for Uber? Slide 14 Translating Business Rules into Data Model Components • Business rules set the stage for the proper identification of entities, attributes, relationships, and constraints – Nouns translate into entities – Verbs translate into relationships among entities • Relationships are bidirectional – Questions to identify the relationship type •How many instance of B are related to one instance of A? •How many instances of A are related to one instance of B? Slide 15 Categories of Data Models • Conceptual (high-level, semantic) data models: – Provide concepts that are close to the way many users perceive data. •(Also called entity-based or object-based data models.) • Physical (low-level, internal) data models: – Provide concepts that describe details of how data is stored in the computer. These are usually specified in an ad-hoc manner through DBMS design and administration manuals • Implementation (representational) data models: – Provide concepts that fall between the above two, used by many commercial DBMS implementations (e.g. relational data models used in many commercial systems). Slide 16 Database Schema • Database Schema: – Represents the logical view of a database. •Consists of tables – Defines how data is organized, the relationship between them, and constraints that might exist. – A good schema can mean the difference between queries lasting a second to hours. • Schema Diagram: – An illustrative display of (most aspects of) a database schema. • Schema Construct: – A component of the schema or an object within the schema, e.g., STUDENT, COURSE. Slide 17 Database State • Database State: – The actual data stored in a database at a particular moment in time. This includes the collection of all the data in the database. – Also called database instance (or occurrence or snapshot). •The term instance is also applied to individual database components, e.g. record instance, table instance, entity instance Slide 18 Database Schema vs Database State • Distinction – The database schema changes very infrequently. – The database state changes every time the database is updated. • Schema is also called intension. – Intension refers to a given relation which is independent of time. An intension defines all permissible extensions. • State is also called extension. – Extension is the state of all records appearing in a database at a given time Slide 19 Example of a Database State Slide 20 Example of a Database Schema Slide 21 Three-Schema Architecture • Proposed to support DBMS characteristics of: – Program-data independence. – Support of multiple views of the data. • Not explicitly used in commercial DBMS products, but has been useful in explaining database system organization Slide 22 Three-Schema Architecture • Defines DBMS schemas at three levels: – Internal schema at the internal level to describe physical storage structures and access paths (e.g indexes). •Typically uses a physical data model. – Conceptual schema at the conceptual level to describe the structure and constraints for the whole database for a community of users. •Uses a conceptual or an implementation data model. – External schemas at the external level to describe the various user views. •Usually uses the same data model as the conceptual schema. Slide 23 Three-Schema Architecture Slide 24 Three-Schema Architecture • Mappings among schema levels are needed to transform requests and data. – Programs refer to an external schema, and are mapped by the DBMS to the internal schema for execution. – Data extracted from the internal DBMS level is reformatted to match the user’s external view (e.g. formatting the results of an SQL query for display in a Web page) Slide 25 Data Independence • Logical Data Independence: – The capacity to change the conceptual schema without having to change the external schemas and their associated application programs. • Physical Data Independence: – The capacity to change the internal schema without having to change the conceptual schema. – For example, the internal schema may be changed when certain file structures are reorganized or new indexes are created to improve database performance Slide 26 Data Independence (continued) • When a schema at a lower level is changed, only the mappings between this schema and higher-level schemas need to be changed in a DBMS that fully supports data independence. • The higher-level schemas themselves are unchanged. – Hence, the application programs need not be changed since they refer to the external schemas. Slide 27 DBMS Languages • Data Definition Language (DDL) • Data Manipulation Language (DML) – High-Level or Non-procedural Languages: These include the relational language SQL •May be used in a standalone way or may be embedded in a programming language – Low Level or Procedural Languages: •These must be embedded in a programming language Slide 28 DBMS Languages • Data Definition Language (DDL): – Used by the DBA and database designers to specify the conceptual schema of a database. – In many DBMSs, the DDL is also used to define internal and external schemas (views). – In some DBMSs, separate storage definition language (SDL) and view definition language (VDL) are used to define internal and external schemas. •SDL is typically realized via DBMS commands provided to the DBA and database designers Slide 29 DBMS Languages • Data Manipulation Language (DML): – Used to specify database retrievals and updates – DML commands (data sublanguage) can be embedded in a general-purpose programming language (host language), such as COBOL, C, C++, or Java. •A library of functions can also be provided to access the DBMS from a programming language – Alternatively, stand-alone
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