Explains join dependency and fifth normal form

K-559 · Knowledge knobit · Activity: Derive the logical data model · Competence: Data modelling

When a three-way entity is the join of its projections, and the cyclic rule that makes it so.

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What it looks like in practice

Expected result: When a three-way entity is the join of its projections, and the cyclic rule that makes it so

How it is assessed: Decides for sample three-way entities whether a cyclic rule holds and whether the entity should be decomposed

Related knobits

Builds on

• K-558 Explains multivalued dependency and fourth normal form

Needed for

• K-513 Recognises a join dependency kept in one entity
• K-523 Recognises a connection trap
• K-560 Explains sixth normal form for temporal data

All 56 knobits of Derive the logical data model.

Source: Normalisation theory and relational design (Codd, Fagin, Date), as used by the stage 5 reference and review checklist