Copyright © 2006 Quest Software

Data Modeling: It’s All About the Relationships

Presenter: Bert Scalzo, Oracle Domain ExpertBert.Scalzo@Quest.comAUDIO INFORMATIONToll free: Toll:Confirmation Code:

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About the Author …Domain Expert & Product Architect for Quest Software

Oracle Background:• Worked with Oracle databases for over two decades (starting with version 4)• Work history includes time at both “Oracle Education” and “Oracle Consulting”

Academic Background:• Several Oracle Masters certifications• BS, MS and PhD in Computer Science• MBA (general business)• Several insurance industry designations

Key Interests:• Data Modeling• Database Benchmarking• Database Tuning & Optimization• "Star Schema" Data Warehouses• Oracle on Linux – and specifically: RAC on Linux

Articles for:• Oracle’s Technology Network (OTN)• Oracle Magazine,• Oracle Informant• PC Week (eWeek)

Articles for:• Dell Power Solutions

Magazine• The Linux Journal• www.linux.com• www.orafaq.com

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Books by Author … Coming in 2008 …

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Agenda

• Purpose– Identify the primary and secondary goals of a data modeling– Speculate as to why data models so often miss primary goal– Propose data modeling tools can’t make up for human error– Examine three well known yet often repeated relationship errors– Provide Best Practices to resolve these issues (i.e. avoid them)

• Overview– No such things “Partial Foreign Keys”– Neither do “Incomplete Foreign Keys”– Unification – what is it and when to do– Conclusion & Questions/Answers

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Partial Foreign Keys Don’t Exist

Q: Why can’t I make certain attributes of a unique identifier relate to the foreign key (FK) in a child any way I like?

A: Because Relational Algebra, Relational Calculus, Codd, Date, and all that’s relational say “Heck No!!!”

Three fundamental FK requirements:• Relation R1 references R2

• R1’s FK attributes have exactly the same domain R2’s PK attributes

• For each tuple (i.e. row) t1 in r(R1) and t2 in r(R2), either t1[FK] = t2[PK] (entirely),or t1[FK] is NULL

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Example #1Identifying

relationship lost

Toad Data Modeler won’t permit this error

Toad Data Modeler makes this less likely

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Example #2Identifying relationship

overuse

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Example #3Identifying relationship

misuse

Toad Data Modeler can workaround error

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Incomplete Foreign Keys Do Not Exist

All or None• Complete PK as the FK (i.e., both columns) or supply the FK with just column A • Not referentially correct, but permitted since column B is optional

Looks Safe• Just as bad or actually worse• Can be legal: A+B, or NULL+NULL• Can be illegal: A+NULL or B+NULL

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Toad Data Modeler will generate check constraint to be safe:

constraint FK_COMPLETEcheck ((A is null and B is null) Or (A is not null and B is not null))

• ANSI SQL 92 permits and databases such as Oracle support alternative matching rules for composite foreign keys, including:

• Match Full –Partially null foreign keys are not permitted. Either all components of the foreign key must be null, or the combination of values contained in the foreign key must appear as the primary or unique key value of a single row of the referenced table.

• Match Partial – Partially null composite foreign keys are permitted. Either all components of the foreign key must be null, or the combination of non-null values contained in the foreign key must appear in the corresponding portion of the primary or unique key value of a single row in the referenced table.

• Match None – Partially null composite foreign keys are permitted. If any column of a composite foreign key is null, then the non-null portions of the key do not have to match any corresponding portion of a parent key.

Incomplete Foreign Keys Do Not Exist

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Unification – not as scary as it sounds!

Q: Why doesn’t Table_3 have columns A and B twice?A: We performed column level unification

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…Otherwise we would have had this – with A & B duplicates in TABLE_3.

Unification – not as scary as it sounds!

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Let’s Test What We’ve Learned – Is This Model Okay?

• FK1 and FK2 both share the column SSN• If employee has doctor’s report, but without associated accident, unifying FK1 and FK2 leads to incomplete FK (i.e., SSN without an accident date)• Left side of Relationship2 is optional – therefore, model is correct

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Thank you!Questions or Comments?

• Three Part Data Modeling Series:– Data Modeling: Common Mistakes and Their Impact– Data Modeling: It’s All About the Relationships– Data Modeling: Reality Requires Supertypes and Subtypes

• Toad Data Modeler v3.2 – Reasonable cost, easy to use data modeling tool for the rest of us …– Support for Oracle 11g and Access 2000-2003, import from SQL script,

SQL code generation by SCHEMA, and much more!– http://www.quest.com/Toad-Data-Modeler