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Principles of Database Design, Conclusions MBAA 609 R. Nakatsu Principles of Database Design Normalization is a process that assigns attributes (fields) to tables such that data redundancies are eliminated or reduced. Four Rules of Thumb: 1. Single Themes: Break up a large table into separate themes, or subjects. 2. Field Uniqueness: Each field in a table should represent a unique type of information (e.g., break up complex fields, and eliminate repeating groups). Principles of Normalization, Continued 3. Primary Keys: Each table must have a unique identifier, or primary key, that is made up of one or more fields in the table. 4. Field Independence: You must be able to make a change to the data in any field (other than a field in the primary key) without affecting the data in any other field. Rule 4: Field Independence Employee (Employee Number, Last Name, First Name, Job Class, Hourly Rate) Employee Number Last Name First Name Job Class Hourly Rate 11 Smith John Mechanic 20 12 Jones Susan Technician 18 13 McKay Bob Mechanic 20 14 Owens Paula Clerk 15 15 Chang Steve Mechanic 20 16 Sarandon Sarah Mechanic 20 In this example, Hourly Rate is dependent on Job Class. Solution: Create Two Tables Employee (Employee Number, Last Name, First Name, Job Class ID) Employee Number Last Name First Name Job Class ID 11 Smith John 2 12 Jones Susan 3 13 McKay Bob 2 14 Owens Paula 1 15 Chang Steve 2 16 Sarandon Sarah 2 Job Class ID is the link to the Job Class table. Job Class (Job Class ID, Job Class, Hourly Rate) Job Class ID 1 2 3 Job Class Clerk Mechanic Technician Hourly Rate 15 20 18 There are no more field dependencies! Integrity of a Database It is important that the data in a database is correct and consistent. Data integrity problems can occur due to: incorrect data entry data redundancy poor database design Ways to Maintain Data Integrity Entity Integrity (primary key must be unique and must not be null) Referential Integrity (see previous slide) Field types (e.g., text, numeric, date, yes/no) Input masks (e.g., ISBN number, dates) Field validation rules (What are some examples?) Good design of input screens (user interfaces) Check digits Check Digit Example Add a check digit to validate a clerk’s data entry. The check digit is determined by some mathematical algorithm. Example: Multiply the first digit by 2; multiply the second digit by 3; multiply the third by 4; add the results; divide by 10 and take the remainder. 127 yields (1*2 + 2*3 + 7*4) / 10 = 36 / 10 for a check digit of 6. Concurrency Control Concurrency control is the management of concurrent transaction execution. Why is it important? The simultaneous execution of transactions over a shared database may create several dataintegrity and consistency problems. Lost Updates: An Example Time 1 2 3 4 5 6 Transaction Step Stored Value T1 Read QOH 50 T2 Read QOH 50 T1 QOH = 50 + 20 T2 QOH = 50 - 30 T1 Write QOH 70 T2 Write QOH 20 Note that the first transaction has not been permanently recorded when the second transaction is executed. Concurrency Control with Locking A lock guarantees exclusive use of a data item to a current transaction. In the previous example, transaction T2 will not have access to the data item that is currently used by transaction T1. The lock is released when the transaction T1 is completed. Data Warehouse stores data that have been extracted from the various operational, external, and other databases of an organization Data warehouses... support OLAP (online analytical processing) support data mining (data in a data warehouse are analyzed to reveal hidden patterns and trends in historical business activity) Data Warehouse Recap Why do we need a database? What is a database? What are the main functions of a DBMS? Entity-Relationship Modeling Linking two tables: 1:1, 1:M, and M:N Four principles of database design Data Integrity Concurrency Control Data Warehouses