Advanced Database Management System - Tutorials and Notes: Description about weak entity sets in DBMS

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Sunday, 31 July 2016

Description about weak entity sets in DBMS

Weak entity sets, Description about weak entity set, Information about weak entity sets, Example weak entity set


Weak Entity Sets


Information about Weak Entity Sets


Properties
Description
Definition
An entity set that may not have sufficient attributes to form a primary key is called as weak entity set.
Association
Every weak entity set must be associated with an identifying strong entity set.
Existence dependent
Weak entity set is said to be existence dependent on the identifying strong entity set.
Owner / Identifying entity set
The entity set on which the weak entity is dependent is the owner or identifying entity set.
Identifying Relationship
The relationship set between the owner and weak entity sets is called identifying relationship.
Type of identifying relationship (Cardinality ratio)
Many-to-One from weak entity set to the owner entity set.
Key of Weak Entity set
The key is called as Discriminator or Partial key.
Participation of weak entity set in the relationship
It is total participation.
ER Diagram Notations
Double rectangle – Weak Entity set
Double diamond – Weak relationship set
Double line – Total participation
Dashed underlined attribute - Discriminator


A weak entity set can participate in other relationships as well.
A weak entity set can have more than one identifying entity sets.

Example:



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 Weak entity set - An example







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