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Database-Management-Systems---10CS54-VTU-notes-->View question

What are structural constraints on relationship types? : Database Management Systems - 10CS54 VTU notes 10 Marks (Dec 14/Jan 15)

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participation: specifies whether or not the existence of an entity depends upon its being related to another entity via the relationship. 
total participation (or existence dependency): 
To say that entity type A is constrained to participate totally in relationship R is to say that if (at some moment in time) R's instance set is { (a1, b1), (a2, b2), ... (am, bm) }, then (at that same moment) A's instance set must be { a1, a2, ..., am }. 
In other words, here can be no member of A's instance set that does not participate in at least one instance of R. According to our informal description of COMPANY, every employee must be assigned to some department. That is, every employee instance must participate in at least one instance of WORKS_FOR, which is to say that EMPLOYEE satisfies the total participation constraint with respect to the WORKS_FOR relationship. In an ER diagram, if entity type A must participate totally in relationship type R, the two are connected by a double line. partial participation: the absence of the total participation constraint! (E.g., not every employee has to participate in MANAGES; hence we say that, with respect to MANAGES, EMPLOYEE participates partially. This is not to say that for all employees to be managers is not allowed; it only says that it need not be the case that all employees are managers. 


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