How to convert Weak entity set into relation schema?
Weak Entity
set is an entity set which does not have enough attributes to form a Primary
key. While converting weak entity set into relation schema, the following
procedure is followed;
- Take the Primary key of Strong entity set in which the Weak entity set depends. Then include the primary key into the relation schema of weak entity set along with other attributes of weak entity set.
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| Figure 1 - ER Diagram for Weak entity and relationships |
Let us
consider the ER diagram given below, where Apartment entity set is the Weak
entity set with the Discriminator ADoor_No. Apartment stores information
regarding the individual apartment details in any building. Hence, a building
can have many floors with more apartments in every floor. Suppose, if we number
every apartment with 1, 2, 3 and so on, then there are other buildings with
same door numbers. Hence, ADoor_No cannot be the Primary key. Apartment
is totally participated on Strong Entity set Building_Details through
the weak relationship set BA for existence. That is, for every
record in Apartment, we must have a related record in Building_details.
The relation
schema Apartment will contain all the attributes of Apartment along with the
Primary key of Strong entity set Building_Details. Thus, we have the following
schema;
Apartment ( Building_No,
ADoor_No, Floor, Rent)
The strong
entity set Building_Details as follows;
Building_Details
(Building_No, Building_Name, Total_Apartments, Address)
Points to note:
The
relationship BA need not be converted into table. In other words, the
relationship between strong and weak entity set is one-to-many. We shall see
that in the post Relationship set into relation schema.