Database design using E-R Model
A database which conforms to an ER diagram can
be represented by a set of tables.
Strong & weak entity
These are two type of entity mainly dependent entity
(also called weak entity) and
independent entity also (called regular
entity and weak entity).
The
dependent entity is the one whose existence dependents on another entity. An entity set is called weak entity set if its
existence depends on the existence of another weak entity or any other strong
entity. A weak entity set does not have sufficient attributes to form a
primary key.
Note => A weak entity
set represented by double outline rectangle in ER diagram.
Consider
an entity type marks which represent the marks obtained by student. Now the
existence of marks dependence upon the existence of entity type student.
&Note
A weak entity set can be converted to strong
entity set by adding appropriate attributes.
Let’s take real life situation to understand
type of entity. Suppose a student says ‘Swati’
can have many instance of marks say eng_marks;
sci_marks; sst_marks etc. related to it if instance ‘Swati’. If Swati is deleted then all the entity marks dependent
upon swati will also removed.
Marks
is dependent (weak) entity that is dependent on entity student.
For example
The entity
Employee & Student are strong entity they have the primary keys Emp_no
& admin no
&Note
An
entity set which has primary key termed as a strong entity set
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