1 Database Systems: Design, Implementation, and Management CHAPTER 6 Database Design

2 The Systems Development Life Cycle l The Systems Development Life Cycle (SDLC) provides a methodology for developing an IS. l Database design takes place within the confines of an IS. l Five phases of SDLC: u Planning u Analysis u Design u Implementation u Maintenance l SDLC is an iterative process

3 Figure 6.2 Systems Development Life Cycle

4 Figure 6.3 Database Lifecycle (DBLC) This is also an iterative process like SDLC

5 Database Design l Divided into four tasks u Conceptual design u DBMS software selection (if required) u Logical design u Physical design l Conceptual design is independent of software and hardware l Logical design is DBMS (software) dependent l Physical design is dependent on both software and hardware

6 Conceptual Design l The goal is to capture and model user requirements l Four Steps: u Data analysis and requirements u Entity relationship modeling and normalization u Data model verification u Distributed database design

7 Conceptual Design l Data analysis and requirements u The focus is on identifying user requirements u This can be gathered through various mean l observing and analyzing the current system l user interviews l questionnaire surveys u Capture and document user data views and business rules. l User data views describe the data used by the user v Example l Business rules describe policies and procedures followed by the company v Example: (EZS) Y An item may be procured from many vendors Y Purchase price of an item is negotiated with each supplier.

8 Conceptual Design l ER Modeling and Normalization u User requirements are modeled using E-R diagrams l Identify main entities based on user requirements data l Define relationships between the entities l Define attributes, primary keys, and foreign keys for each of the entities. l Normalize the entities. l Complete the initial E-R diagram. l Verify the E-R model against the data, information, and processing requirements. l Modify the E-R diagram, if necessary u Documentation process must be standardized to avoid miscommunication

9 Conceptual Design l Data model verification u Ensure that user data views can be supported by the data model u All business transactions (select, insert, update, delete, user queries) can be supported by the model l Distributed database design u Data requirements and processing requirements may vary from one location to another u Decision may be made about allocating data to different locations

10 DBMS Selection l This step is required only if you plan to acquire a new DBMS l Common factors affecting the decision: u Cost -- Purchase, maintenance, operational, license, installation, training, and conversion costs. u DBMS features and tools. u Underlying model. u Portability -- Platforms, systems, and languages. u DBMS hardware requirements.

11 Logical Design l Logical design translates the conceptual design into the internal model for a selected DBMS. l It includes the design of tables, indexes, views, transactions l Access authorities (who can access what) are also decided. l The ER model is translated into relational schema

12 Logical Design l Translating ER Model into Relational Schema u After normalizing the E-R diagram we are left with only two types of relationships l One-to-one l One-to-Many u For every one-to-one relationship, reexamine the possibility of merging the two entities into a single entity by combining their attributes. u Entities participating in a one-to-one relationship are linked through a foreign key. u Supertype-subtype relationships are usually implemented as one-to-one relationships. Both entities share a common primary key, which also becomes a foreign key in the subtype entity.

13 Logical Design EmployeeDriver May be a 11 (0,1)(1,1) Employee Emp_Id Emp_Name Emp_Salary Driver Emp_Id License Nbr Lic Exprn. Date 11 Example of translating a 1:1 relationship into a relational schema Primary and Foreign Key

14 Logical Design l Translating ER Model into Relational Schema u One-to-many relationships are implemented by adding the primary key of the first entity as the foreign key of the second (many side) entity. ProfessorClass teaches 1M (0,N)(1,1) Professor Prof_Id Prof_Lname Prof_Phone Class Class_Code Class_Section Class_Days Class_Time Prof_Id 1 M Example: Foreign Key

15 Example - Logical Design

16 Example - Logical Design

17 Physical Design l Select data storage and data access characteristics (indexes) of the database. l It affects location of the data in the storage device(s) and system performance. l Physical design is more complex with distributed databases. l Relational databases are more insulated from physical layer details than hierarchical and network models. l Chapters 7 and 8 describe an excellent case study of database design

18 Physical Organization Figure 6.12

19 DB Design Strategy Notes l Top-down u 1) Identify data sets u 2) Define data elements l Bottom-up u 1) Identify data elements u 2) Group them into data sets Figure 6.14