A weak entity cannot be used independently as it is dependent on a strong entity type known as its owner entity. Also, the relationship that connects the weak entity to its owner identity is called the identifying relationship.
A weak entity always has a total participation constraint with respect to its identifying relationship because it cannot be identified independently of its owner identity.
A weak entity may have a partial key, which is a list of attributes that identify weak entities related to the same owner entity.
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.
In the given ER diagram, Dependent is the weak entity and it depends on the strong entity Employee via the relationship Depends on.
There can be an employee without a dependent in the Company but there will be no record of the Dependent in the company systems unless the dependent is associated with an Employee.
A strong entity is complete by itself and is not dependent on any other entity type. It possess a primary key which describes each instance in the strong entity set uniquely. That means any element in the strong entity set can be uniquely identified.
A Strong entity is represented by a square with a single line unlike a Weak Entity which contained double lines.