What is the defining attribute for an entity?
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My question is whether one of the defining attributes could be null?
We can identify a Quote with 'Base Asset + Trade on Market + Quotation Currency + Trade on Sub Market'. When Trade on Sub Market is nullable, whether Trade on Sub Market can be a defining attribute?
Quote from the principal as below.
Statement:
AnInformation object must not be created nor registered without a set ofattribute values which can be used to uniquely differentiate its existence.
Rationale:
Where there are multiple sources or there is a reliance on 3rdparty content for an information object type, there must be the ability toconcord them and to highlight and reconcile inconsistencies. This will preventcreation of duplicate instances by the Authority.
Implications:
Two approaches are recognised in the Content Marketplace:
1. Defining attributes - A set of attributes are selected and used to the same effect as a compositeprimary key. No two registered instances of an object type can have the sameset of defining attribute values. All registered instances are therefore knownto be different.
2. Uniqueness attributes - A set of attributes are selected to be used as the basis forselecting instances (by search or matching algorithm for example). Not allattributes need be present or used, although the more used, the more unique theselection will be. Selection results will comprise a set of instances matchingthe uniqueness attributes used: human or AI intervention may be needed to pickthe intended target from the result set. All registered instances are believeddifferent based on that intervention.
When registering new objects, in both cases it is the act of allocation of a uniquePermID to the registered instance that confers uniqueness on that instance. Aunique combination of defining attributes leads directly to the need for a new PermID; using uniqueness attributes often requires an intervention to ascertainwhether a new PermID is needed.
From above description, we can see the defining attributes is in fact a combination PK.
the unique attributes is in fact a combination FK.
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