A weak entity is something that cannot exist without another entity. It depends on a strong entity.
A partial key is a part of the weak entity that helps identify each dependent for the same employee.
But a partial key alone is not enough. It only works when combined with the employee’s ID.
In this case, the Date of Birth (DOB) of the dependent is used as a partial key, because usually, no two dependents of the same employee are born on the same date. And therefore DOB was the best option for selecting the partial key.
So, EmployeeID + Dependent DOB can uniquely identify each dependent.
That’s why DOB is called a partial key — it helps only when combined with EmployeeID.
I. Dependent is a weak entity (bcz it depends on the Employee table)
II. DOB is a partial key (bcz it's not enough by itself, but works as a primary key with E_ID).
III. primary key = (EmployeeID + DOB) becomes the primary key for the Dependent table.