Comparison between Strong and Weak Entity
The Key Difference between Strong and Weak Entity is that a Strong Entity has a primary key whereas a Weak Entity does not have a primary key.
Weak entity is dependent on a strong entity whereas a Strong entity is independent not dependent on any other.
|Strong Entity||Weak Entity|
|1||Strong entity has a primary key.||Weak entity has a partial key.|
|2||Strong entity is independent||Weak entity is dependent on a strong entity|
|3||Strong entity indicated by a single rectangle.||Strong entity indicated by a double rectangle.|
|4||Two strong entity’s relationship is indicated by a single diamond.||One strong and one weak entity is indicated by a double diamond.|
|5||Strong entity may be or may not be participate relationships.||Weak entity always participates relationships.|
|6||In strong entity connecting line is a single line||In weak entity connecting line is a double line|
- An entity set that does not have a primary key is called a weak entity set.
- The existence of a weak entity set depends on the existence of a strong entity set.
- Weak entity set is indicated by a double rectangle.
- Weak entity relationship set is indicated by a double diamond.
- The discriminator (partial key) of a weak entity set is the set of attributes that distinguishes between all the entities of a weak entity set.
- The primary key of a weak entity set is created by combining the primary key of the strong entity set on which the weak entity set is existence dependent and the weak entity set’s discriminator.
- Underline the discriminator attribute of a weak entity set with a dashed line.
- Example is the below image.
- There are two entities loan and payment in which loan is strong entity set and payment is weak entity set.
- Payment entity has payment-no which is the discriminator.
- Loan entity has loan-no as a primary key.
- So the primary key for payment is (loan-no, payment-no).
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