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What is a “functional dependency”? How does it relate to database table design?
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What is a “functional dependency”? How does it relate to database table design?
3,457 Views | (13 votes, avg: 4.15)
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Functional dependency relates to how one object depends upon the other in the database.
for example, procedure/function sp2 may be called by procedure sp1. Then we say that sp1 has functional dependency on sp2.
Functional Dependence (FD): Given an enity E, attribute B of E is functionally dependent on attribute A of E if and only if each value of A in E has associated with it precisely one value of B in E(at any one time). In other words, A uniquely determines B.
Full Funtional Dependence: Given an entity E, an attribute B of E is fully functionally dependent on a set of attributes A of E if and only if B is functinally dependent on A and not functionally dependent on any proper subset of A .
Pandian , would you please help me understand your answer better by giving an example ? Thank you very much.
What functional dependence in the context of a database means is that :
Assume that a table exists in the database called TABLE with a composite primary key (A, B) and other non-key attributes (C, D, E). Functional dependency in general, would mean that any non-key attribute - C D or E being dependent on the primary key (A and B)in our table here.
Partial functional dependency, on the other hand, is another corollary of the above, which states that all non-key attributes - C D or E - if dependent on the subset of the primary key (A and B) and not on it as a whole.
Example :
———-
Fully Functional Dependent : C D E –> A B
Partial Functional dependency : C –> A
D E –> B
Hope that helps!
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.”
Ex:-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.
In an entity set called Employee having attributes emp.name,emp.id,emp.address.If someone says that attribute emp.name is functional dependency on attribute emp.id,it means for each value of attribute emp.id there is exactly one value of emp.name(note that reverse need not be true:as several employees could have same name which corresponds to different employees id.)
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