# Difference between Strong and Weak Entity

## 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.

### Comparison Chart

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

### Weak Entity

• 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).