Relationships

The [Missing] Link

Michael P. Meier

WhiteLake Data Management

What I’m Going to Tell You

Data Architects ignore relationshipsThis includes all types– Inter-personal– Logical– Physical (data)

We’re suffering without relationshipHere’s what we need to do.

Modeling Formalisms

ERA (entity-relationship-attribute and variants)

UML (unified modeling language)ORM (object-role model)

None today give more than lip service

It wasn’t always so

Peter Chen (1976)* represented relationships as diamond-shaped

Two-dimensional symbols gave relationships equal weight with entities

3NF relational schemas were the goal

*1976 article published in ACM’s Transactions on Database Systems

Chen Diagram

doctor doctor

consultation

Dr-pt

residency

Chen Diagram Suggests

Three different relationships exist between two doctors

An implementation consisting of three tables to represent the relationships

The relationships may have additional properties that are different– Dates (start, end)– Relationships to other entities– Rotation, Diagnosis, Recommendation,…

“Modern” Interpretation

Doctor Doctor

or

Doctor

We usually don’t even get names on the “relationships.”

Which Suggests

Three foreign key references to Doctor within the Doctor table

Meaning that– Additional relationships and attributes

are lost.– Or, 3NF is lost (by including

information about the relationship in the Doctor entity)

More Importantly

The relationship is a detail– A connector on a diagram

When it should be a core piece of intelligence about the business

The majority of business rules are about relationships.

Models and applications suffer from trivializing relationships

Even Worse

Doctor

This is what we usually see.

This is the barest hint about the possible existence of relationships

Effect on Data Quality

Inattention to relationships leads to the worst sort of data quality problem.

The “I meant” problem– A foreign key column in a table contains

values that share• Data type• Domain• Range

– But were placed there for different reasons• Therefore have different semantic values

Undefined Relationships

Result in poorly designed user interfacesUndocumented rulesReporting nightmaresData degradationUnattributed or improperly attributed costs

to the business

Suggestions Always name relationships—including role names. Supply a description for the relationship Always model relationships as entities. Investigate them. They probably have attributes and

possibly relationships.– Events are really relationships– Often n-ary relationships (more than two participants)

Think, really think, before you “denormalize” your physical model– At least start with a normalized model– Many use “denormalize” as an excuse not to do the

work.

There is nothing simple about a relationship.

Ask your spouse, significant other, friends, neighbors…

Relationships are continuously being renegotiated (changing).

Give them attention up front or pay dearly later.