Difference Between Strong Entity Set And Weak Entity Set In DBMS (With Examples)

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What Is An Entity?

An entity set is a collection or a set of all the entities that share the same attributes set but with a different set of values for some attributes. The entity can be classified into two basic categories:

  • Weak Entity
  • Strong Entity

What Is a Strong Entity?

A strong entity is an entity that is independent of any other entity in a schema. It has sufficient attributes to uniquely identify all its entities. Strong entity has a primary key attribute or a table which consists of a primary key. The primary key of the strong entity is represented by underlining it.

In as far as representation is concerned; a strong entity is represented by a single rectangle whereas the relationship between two entities is represented by a single diamond. 

What You Need To Know About Strong Entity Set

  • A strong entity is an entity that is independent of any other entity in a schema. It has sufficient attributes to form a primary key.
  • The member of a strong entity set is referred to as a dominant entity set.
  • A strong entity is denoted with a single rectangle.
  • Strong entity is not dependent on any other entity in a schema.
  • The primary key is one of its attributes which uniquely identifies its member.
  • A strong entity always has a primary key represented by an underline.
  • In the ER diagram, the relationship between two strong entity set is represented by a diamond symbol.
  • The connecting line of the strong entity set with the relationship is single.
  • The strong entity may or may not show the total participation in its relations.

Example Of Strong Entity Set

  • Consider the ER diagram which consists of two entity Customer and account
  • Customer entity is a strong entity because it consists of a primary key called customer ID which is enough for accessing each record uniquely.
  • The same way, account entity contains of account ID attribute which is capable of uniquely accessing each row with each customer details.

What Is a Weak Entity Set?

A weak entity is an entity set that does not have sufficient attributes for unique identification of its records. A weak entity does not have a primary key attribute; it contains a partial key referred to as discriminator which helps in identifying a group of entities from the entity set. The discriminator is represented by underlining with a dashed line.

In as far as representation is concerned; a double rectangle is used for representing a weak entity set whereas double lines are used for presenting the connection with a weak entity set with relationship. Also, the double diamond symbol is used for representing the relationship between a strong entity and weak entity which is known as identifying relationship.

What You Need To Know About Weak Entity

  • A weak entity is an entity set that cannot be uniquely identified by its attributes alone. It does not have sufficient attributes to form a primary key.
  • The member of a weak entity set is referred to as a subordinate entity set.
  • A weak entity is denoted with a double rectangle.
  • Weak entity is dependent on strong entity for its existence.
  • The primary key of the weak entity set is a combination of partial key and primary key of the strong entity.
  • A weak entity has a partial key or discriminator, which is a list of attributes that identify weak entities related to the same owner entity. The partial key is represented by a dashed underline.
  • In the ER diagram, both the weak entity and its corresponding relationship are represented using a double line and the partial key is underlined with a dotted line.
  • The line connecting the weak entity set for identifying relationship is double.
  • Weak entity has a total participation constraint with respect to its identifying relationship because it cannot be identified independently of its own identity.

Example of a Weak Entity

  • In the ER diagram, we have two entities building and apartment. Building is a strong entity because it has a primary key attribute referred to as building number which is capable of uniquely identifying all the flats present in the apartment.
  • Unlike building, apartment is weak entity because it does not have any primary key and door number here acts only as a discriminator because door number cannot be used as a primary key, there might be multiple flats in the building with the same door number or on different floors.

Difference Between Strong Entity  And Weak Entity In Tabular Form

BASIS OF COMPARISON STRONG ENTITY WEAK ENTITY  
Description A strong entity is an entity that is independent of any other entity in a schema. It has sufficient attributes to form a primary key.   A weak entity is an entity set that cannot be uniquely identified by its attributes alone. It does not have sufficient attributes to form a primary key.  
Name For Member The member of a strong entity set is referred to as a dominant entity set.   The member of a weak entity set is referred to as a subordinate entity set.  
Symbol A strong entity is denoted with a single rectangle.   A weak entity is denoted with a double rectangle.  
Dependency Strong entity is not dependent on any other entity in a schema.   Weak entity is dependent on strong entity for its existence.  
Primary Key Structure The primary key is one of its attributes which uniquely identifies its member.   The primary key of the weak entity set is a combination of partial key and primary key of the strong entity.  
Primary Key A strong entity always has a primary key represented by an underline.   A weak entity has a partial key or discriminator, which is a list of attributes that identify weak entities related to the same owner entity. The partial key is represented by a dashed underline.  
ER Diagram In the ER diagram, the relationship between two strong entity set is represented by a diamond symbol.   In the ER diagram, both the weak entity and its corresponding relationship are represented using a double line and the partial key is underlined with a dotted line.  
Connection Line The connecting line of the strong entity set with the relationship is single.   The line connecting the weak entity set for identifying relationship is double.  
Total Participation The strong entity may or may not show the total participation in its relations.   Weak entity has a total participation constraint with respect to its identifying relationship because it cannot be identified independently of its own identity.