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Graded Discussion Board (GDB) of CS403 - Database Management Systems will be launched on 19th of February 2014 (19-Feb-2014) and will last for 48 hours only till (20-Feb-2014) 11:59 PM

GDB_FALL 2013 Dated: Feb 14, 14

Dear Students!

Please Note that GDB of CS403 will be launched on 19th of February 2014 (19-Feb-2014) and will last for 48 hours only.

The topic will be as follows:

Suppose there are two entities "building" and "apartment". The entity "apartment" is dependent entity and cannot exist independently and “building” is an independent entity. The "apartment" entity contains the primary key (building_name) of building entity to identify the apartment of that specific building. The "apartment" entity also has its own identifier, i.e. apartment_no but that identifier is not sufficient to identify the particular apartment, so in this case we used both keys building_name and apartment_no to identify the particular apartment.

 

You have studied different dependencies in the course. You are required to choose the appropriate dependency applicable in  the above scenario. Justify your answer with reasons.

Try to provide precise, to the point answer and avoid irrelevant details.

 

For any query: CS403@vu.edu.pk

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Replies to This Discussion

Taria Bahi exect btein kon c dependency hai

 A functional dependency occurs when one attribute in a relation uniquely determines another attribute. This can be written A -> B which would be the same as stating "B is functionally dependent upon A."

Examples:

In a table listing employee characteristics including Social Security Number (SSN) and name, it can be said that name is functionally dependent upon SSN (or SSN -> name) because an employee's name can be uniquely determined from their SSN. However, the reverse statement (name -> SSN) is not true because more than one employee can have the same name but different SSNs.

A partial functional dependency is a functional dependency where the determinant consists of key attributes, but not the entire primary key, and the determined consist~ of non-key attributes. 

A transitive functional dependency is a functional dependency where the determinant consists of non-key attributes and the determined also consists of non-key attributes. 

A Boyce-Codd functional dependency is a functional dependency where the determinant consists of non-key attributes and 

Type of Functional Dependence (FD)

A functional dependency is an association between two attributes of the same relational database table. One of the attributes is called the determinant and the other attribute is called the determined. For each value of the determinant there is associated one and only one value of the determined.


If A is the determinant and B is the determined then we say that A functionally determines B and graphically represent this as A -> B. The symbols A à B· can also be expressed as B is functionally determined by A.



Referential Dependency:

This is the situation when the dependent entity has it own key for unique identification but the key used to show the reference with the parent entity is shown with the help of an attribute of the parent entity. Means to show the link of the parent entity with this entity there will be an attribute and a record in this entity will not exist without having a record in the parent entity

A partial functional dependency is a functional dependency where the determinant consists of key attributes, but not the entire primary key, and the determined consist~ of non-key attributes.

A transitive functional dependency is a functional dependency where the determinant consists of non-key attributes and the determined also consists of non-key attributes.

A Boyce-Codd functional dependency is a functional dependency where the determinant consists of non-key attributes and the determined consists of key attributes.

Importance of Dependencies

Database dependencies are important to understand because they provide the basic building blocks used in database normalization. For example:

 

    For a table to be in second normal form (2NF), there must be no case of a non-prime attribute in the table that is functionally dependent upon a subset of a candidate key.
    For a table to be in third normal form (3NF), every non-prime attribute must have a non-transitive functional dependency on every candidate key.
    For a table to be in Boyce-Codd Normal Form (BCNF), every functional dependency (other than trivial dependencies) must be on a super key.
    For a table to be in fourth normal form (4NF), it must have no multivalued dependenci


Full Functional Dependencies

A full functional dependency occurs when you already meet the requirements for a functional dependency and the set of attributes on the left side of the functional dependency statement cannot be reduced any farther. For example, “{SSN, age} -> name” is a functional dependency, but it is not a full functional dependency because you can remove age from the left side of the statement without impacting the dependency relationship.

Transitive Dependencies

Transitive dependencies occur when there is an indirect relationship that causes a functional dependency. For example, ”A -> C” is a transitive dependency when it is true only because both “A -> B” and “B -> C” are true.

Multivalued Dependencies

Multivalued dependencies occur when the presence of one or more rows in a table implies the presence of one or more other rows in that same table. For example, imagine a car company that manufactures many models of car, but always makes both red and blue colors of each model. If you have a table that contains the model name, color and year of each car the company manufactures, there is a multivalued dependency in that table. If there is a row for a certain model name and year in blue, there must also be a similar row corresponding to the red version of that same car.

Transitive Dependency
Transitive dependency is one that carries over another attribute. Transitive dependency occurs when one non-key attribute determines another non-key attribute. For third normal form we concentrate on relations with one candidate key, and we eliminate transitive dependencies. Transitive dependencies cause insertion, deletion, and update anomalies.

Definition of Functional Dependency
If A and B are attributes or sets of attributes of relation R, we say that B is functionally dependent on A if each value of A in R has associated with it exactly one value of B in R. We write this as A B, read as “A functionally determines B” or “ A determines B”.

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