Recognises a lossy decomposition

K-514 · Skill knobit · Activity: Derive the logical data model · Competence: Data modelling

An entity is split so that joining the parts back produces rows that never existed. The split is lossless only if the shared attributes are a key of at least one part.

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What it looks like in practice

Expected result: The split, the shared attributes that are not a key, and an example spurious row

How it is assessed: Flags the deficient case and passes the acceptable one. Deficient: Account Holder split into (Account, Holder Role) and (Party, Holder Role) and joined on Holder Role: two accounts each with one joint holder give four holdings Acceptable: Keep (Account, Party, Holder Role) whole, or split only on attributes that are a key of one side

Also known as: Lossy decomposition (spurious rows) (logical model)

Related knobits

Builds on

• K-563 Explains lossless-join decomposition

Needed for

• K-604 Recognises a lossy or unconstrained vertical split

All 56 knobits of Derive the logical data model.

Source: Review checklist 05-logical, row 14; Heath 1971; E&N 14.1.4, ch. 15