Data Modeling: It’s All
About the Relationships
Presenter: Bert Scalzo, Oracle Domain Expert
Bert.Scalzo@Quest.com
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Copyright © 2006 Quest Software
About the Author …
Domain Expert & Product Architect for Quest Software
Oracle Background:
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Worked with Oracle databases for over two decades (starting with version 4)
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Work history includes time at both “Oracle Education” and “Oracle Consulting”
Academic Background:
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Several Oracle Masters certifications
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BS, MS and PhD in Computer Science
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MBA (general business)
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Several insurance industry designations
Key Interests:
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Data Modeling
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Database Benchmarking
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Database Tuning & Optimization
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"Star Schema" Data Warehouses
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Oracle on Linux – and specifically: RAC on Linux
Articles for:
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Oracle’s Technology Network (OTN)
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Oracle Magazine,
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Oracle Informant
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PC Week (eWeek)
Articles for:
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Dell Power Solutions
Magazine
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The Linux Journal
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www.linux.com
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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 #1
Identifying
relationship lost
Toad Data Modeler
won’t permit this error
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Example #2
Identifying relationship
overuse
Toad Data Modeler
makes this less likely
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Example #3
Identifying 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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Incomplete Foreign Keys Do Not Exist
Toad Data Modeler will generate check constraint to be safe:
constraint FK_COMPLETE
check ((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.
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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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Unification – not as scary as it sounds!
…Otherwise we would have had this – with A & B
duplicates in TABLE_3.
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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
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