⇒ A strong entity always has a primary key and does not depend on any other entity for its existence.
⇒ Represented by a single rectangle as shown in below fig.
⇒ A weak entity does not have enough attributes to form a primary key. It depends on a strong entity to ensure its existence.
⇒ Represented by a double rectangle as shown in below fig.
The existence of a weak entity set depends on the existence of a strong entity set (called the identifying entity set).
The relationship that associates the weak entity set with the strong entity set is called the identifying relationship, represented by a double diamond.
The identifying relationship is a one-to-many relationship from the strong entity set to the weak entity set (1-M relationship). For example, one Loan can have many Payments.
The discriminator is a set of attributes that distinguishes among all the entities in a weak entity set. It is underlined with a dashed line.
The primary key of a weak entity set is formed by combining the primary key of the strong entity set and the discriminator of the weak entity set.
Example
The Professor is a Strong Entity, whereas Dependent is a Weak Entity.
ID is the Primary key (represented with a line) and Name
in Dependent entity is called Partial Key (represented with a dotted line).
