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UT Dallas CS 6360 - Ch15_FDs-NormalForms

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Chapter 15Chapter OutlineSlide 3Informal Design Guidelines for Relational Databases (1)Informal Design Guidelines for Relational Databases (2)Semantics of the Relation AttributesSlide 7Redundant Information in Tuples and Update AnomaliesEXAMPLE OF AN UPDATE ANOMALYEXAMPLE OF AN INSERT ANOMALYEXAMPLE OF AN DELETE ANOMALYSlide 12Slide 13Guideline to Redundant Information in Tuples and Update AnomaliesNull Values in TuplesSpurious TuplesSpurious Tuples (2)2.1 Functional Dependencies (1)Functional Dependencies (2)Examples of FD constraints (1)Examples of FD constraints (2)Slide 22Normal Forms Based on Primary KeysNormalization of Relations (1)Normalization of Relations (2)Practical Use of Normal FormsDefinitions of Keys and Attributes Participating in KeysSlide 28First Normal FormSlide 30Slide 31Second Normal FormSlide 33Slide 34Third Normal FormSlide 36Slide 37Normal Forms Defined InformallySlide 39General Normal Form Definitions (For Multiple Keys)General Normal Form DefinitionsBCNF (Boyce-Codd Normal Form)Slide 43Chapter SummaryChapter 15Functional Dependencies and Normalization for Relational Databases2Chapter Outline1. Informal Design Guidelines for Relational Databases1.1 Semantics of the Relation Attributes1.2 Redundant Information in Tuples and Update Anomalies1.3 Null Values in Tuples1.4 Spurious Tuples2. Functional Dependencies (FDs)3Chapter Outline3 Normal Forms Based on Primary Keys3.1 Normalization of Relations 3.2 Practical Use of Normal Forms 3.3 Definitions of Keys and Attributes Participating in Keys 3.4 First Normal Form3.5 Second Normal Form3.6 Third Normal Form4 General Normal Form Definitions (For Multiple Keys)5 BCNF (Boyce-Codd Normal Form)4Informal Design Guidelines for Relational Databases (1)What is relational database design?The grouping of attributes to form "good" relation schemas Two levels of relation schemasThe logical "user view" levelThe storage "base relation" level Design is concerned mainly with base relations What are the criteria for "good" base relations?5Informal Design Guidelines for Relational Databases (2)We first discuss informal guidelines for good relational designThen we discuss formal concepts of functional dependencies and normal forms- 1NF (First Normal Form)- 2NF (Second Normal Form)- 3NF (Third Normal Form)- BCNF (Boyce-Codd Normal Form)6Semantics of the Relation Attributes GUIDELINE 1: Informally, each tuple in a relation should represent one entity or relationship instance. (Applies to individual relations and their attributes).Attributes of different entities (EMPLOYEEs, DEPARTMENTs, PROJECTs) should not be mixed in the same relationOnly foreign keys should be used to refer to other entitiesEntity and relationship attributes should be kept apart as much as possible.Bottom Line: Design a schema that can be explained easily relation by relation. The semantics of attributes should be easy to interpret.A simplified COMPANY relational database schema8Redundant Information in Tuples and Update Anomalies Information is stored redundantly Wastes storageCauses problems with update anomaliesInsertion anomaliesDeletion anomaliesModification anomalies9EXAMPLE OF AN UPDATE ANOMALYConsider the relation:EMP_PROJ(Emp#, Proj#, Ename, Pname, No_hours)Update Anomaly:Changing the name of project number P1 from “Billing” to “Customer-Accounting” may cause this update to be made for all 100 employees working on project P1.10EXAMPLE OF AN INSERT ANOMALYConsider the relation:EMP_PROJ(Emp#, Proj#, Ename, Pname, No_hours)Insert Anomaly:Cannot insert a project unless an employee is assigned to it.ConverselyCannot insert an employee unless a he/she is assigned to a project.11EXAMPLE OF AN DELETE ANOMALYConsider the relation:EMP_PROJ(Emp#, Proj#, Ename, Pname, No_hours)Delete Anomaly:When a project is deleted, it will result in deleting all the employees who work on that project.Alternately, if an employee is the sole employee on a project, deleting that employee would result in deleting the corresponding project.12Two relation schemas suffering from update anomalies13Base Relations EMP_DEPT and EMP_PROJ formed after a Natural Join : with redundant information14Guideline to Redundant Information in Tuples and Update AnomaliesGUIDELINE 2: Design a schema that does not suffer from the insertion, deletion and update anomalies.If there are any anomalies present, then note them so that applications can be made to take them into account.15Null Values in Tuples GUIDELINE 3:Relations should be designed such that their tuples will have as few NULL values as possibleAttributes that are NULL frequently could be placed in separate relations (with the primary key) Reasons for nulls:Attribute not applicable or invalidAttribute value unknown (may exist)Value known to exist, but unavailable16Spurious Tuples Bad designs for a relational database may result in erroneous results for certain JOIN operationsThe "lossless join" property is used to guarantee meaningful results for join operations GUIDELINE 4:The relations should be designed to satisfy the lossless join condition.No spurious tuples should be generated by doing a natural-join of any relations.17Spurious Tuples (2)There are two important properties of decompositions: a) Non-additive or losslessness of the corresponding joinb) Preservation of the functional dependencies. Note that:Property (a) is extremely important and cannot be sacrificed.Property (b) is less stringent and may be sacrificed. (Chapter 16).182.1 Functional Dependencies (1) Functional dependencies (FDs)Are used to specify formal measures of the "goodness" of relational designsAnd keys are used to define normal forms for relationsAre constraints that are derived from the meaning and interrelationships of the data attributesA set of attributes X functionally determines a set of attributes Y if the value of X determines a unique value for Y19Functional Dependencies (2)X -> Y holds if whenever two tuples have the same value for X, they must have the same value for YFor any two tuples t1 and t2 in any relation instance r(R): If t1[X]=t2[X], then t1[Y]=t2[Y]X -> Y in R specifies a constraint on all relation instances r(R)Written as X -> Y; can be displayed graphically on a relation schema as in


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UT Dallas CS 6360 - Ch15_FDs-NormalForms

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