Database-MySQL [3i Infotech Placement]: Sample Questions 157 - 157 of 162

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Question 157


Describe in Detail


What is Weak Entity set?


Weak Entities Set with Example
  • An entity set that does not have a primary key is referred to as a weak entity set.
  • The existence of a weak entity set depends on the existence of a identifying entity set:
    • It must relate to the identifying entity set via a total, one-to-many relationship set from the identifying to the weak entity set.
    • Identifying relationship depicted using a double diamond.
  • The discriminator (or partial key) of a weak entity set is the set of attributes that distinguishes among all the entities of a weak entity set.
  • The primary key of a weak entity set is formed by the primary key of the strong entity set on which the weak entity set is existence dependent, plus the weak entity set՚s discriminator.
  • We depict a weak entity set by double rectangles.
  • We underline the discriminator of a weak entity set with a dashed line.

    Example of weak Entity set.

Example of Weak Entity Set
  • If Name were explicitly stored, Dependent could be made a strong entity, but then the relationship between student and depends would be duplicated by an implicit relationship defined by the attribute Name common to student and depend.

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