answersLogoWhite

0

A superkey is defined as a set of attributes of a relation for which it holds that in all relations assigned to

that variable there are no two distinct tuples (rows) that have the same values for the attributes in this set.

As an example

Code:

Roll Number | First Name | Last Name

CSU0001 | Shabbir | Bhimani |

CSU0002 | SomeName | SurName |

CSU0003 | Larry | page |

Now here we have the following as super keys

1. Roll Number | First Name

2. Roll Number | First Name | Last Name

Now in plain language Any Unique key with some Non Unique key combination is called a super key of the

relationship.

A candidate key of a relationship is a set of attributes of that relationship such that there are no two distinct

tuples with the same values for these attributes. In simple example candidate key is a minimal superkey, i.e. a

superkey of which no proper subset is also a superkey.

Since a relation is a set(no duplicate elements), it holds that every relation will have at least one candidate key

(because the entire heading is always a superkey). For practical reasons RDBMSs usually require that for each

relation one of its candidate keys is declared as the primary key

Quote:

For example, Given an employee table consisting of the columns:

employeeID

name

job and

departmentID

we could use the employeeID in combination with any or all other columns of this table to uniquely identify a row

in the table. Examples of superkeys in this table would be {employeeID, Name}, {employeeID, Name, job}, and

{employeeID, Name, job, departmentID}.

In a real database we don't need values for all of those columns to identify a row. We only need, per our example,

the set {employeeID}. This is a minimal superkey - that is, a minimal set of columns that can be used to identify a

single row. So, employeeID is a candidate key.

Now, if employeeID is a candidate key then why not it is the superkey. Because employeeID can also uniquely

identify the tuples.

(2) In your example why Roll number is not the superkey as it is uniquely identifying the tuples?

(3)

Quote:

Any Unique key with some Non Unique key combination is called a super key of the relationship

Is it necessary that Unique key has to be combined with some Non Unique key to be called as a super key.

Unique key is also a super key but the minimal super key is called candidate key and all candidate keys are super

keys but the reverse is not true.

User Avatar

Wiki User

13y ago

What else can I help you with?

Continue Learning about Algebra
Related Questions

What is the difference between a key and a superkey?

In database a key is a field that we use to sort a data.whereas superkey is a subset of attribute so that no tuple have the same combination of values.


What is a minimal superkey?

A minimal superkey, also known as a candidate key, is a type of superkey that uniquely identifies a record in a database table while having no unnecessary attributes. In other words, it is a superkey from which no attribute can be removed without losing its uniqueness property. Every minimal superkey is a superkey, but not all superkeys are minimal, as superkeys may contain additional attributes that are not needed for unique identification.


Difference between a super key and a candidate key?

Different set of attributes which are able to identify any row in the database is known as super key. And minimal super key is termed as candidate key i.e. among set of super keys one with minimum number of attributes. Primary key could be any key which is able to identify a specific row in database in a unique manner.


Is super set of the super key is the super key.?

A superkey is essentially a super set of a key. Consider a relation R(a,b,c,d) where {a} is the key. Any addition to a such as {a,b},{a,c}, {a,b,c} are considered superkeys. Furthermore, if you add more attributes to a superkey, it will be considered a superkey as well. So yes, a superset of a superkey is another superkey.


What is the prime attribute in DBMS?

prime attributes are the attributes of a candidate key which will give uniqueness. a candidate key is (the subset of superkey) which gives uniqueness. Super key is the maximum set of attributes that can provide uniqueness example ABCDE is a super key AB CD DE are the candidate keys s.t. AB is the primary key. CD and DE are the secondary/alternate keys.


What is the difference between primary key and candidate key?

Candidate Key is used to uniquely identify the records of a table. An attribute becomes a Primary Key, if all the other candidate keys lose race for being qualified as Primary Key.


What is difference between primary candidate and candidate key?

don't ask me I'm trying to find out to but i think it has something to do with the government


Definition of super key with example?

A superkey is a combination of attributes that can be uniquely used to identify a database record. A table might have many superkeys. Candidate keys are a special subset of superkeys that do not have any extraneous information in them.


What is the difference between a key and a superkey in DBMS?

I am supposing that key means primary key.Superkey is any subset of attributes that uniquely identifies the tuples of a relation. This subset need not be minimal, that is, one can remove some attributes from it and it is still uniquely identifying. If all redundant attributes are removed you get a subset what is called primary key.


What is the difference between a candidate key an a secondary key?

A candidate key is a unique key that can uniquely identify each record in a database table, whereas a secondary key is a non-unique key used for data retrieval and indexing purposes. Candidate keys can be chosen as the primary key for the table, while secondary keys provide alternate ways to access and organize the data.


What are the key differences between a PhD student and a PhD candidate?

The key difference between a PhD student and a PhD candidate is that a PhD student is still completing coursework and working on their research, while a PhD candidate has finished their coursework and is focused on completing their dissertation. In other words, a PhD candidate is further along in the process of earning their PhD than a PhD student.


All types of keys in database?

* Alternate key - An alternate key is any candidate key which is not selected to be the primary key * Candidate key - A candidate key is a field or combination of fields that can act as a primary key field for that table to uniquely identify each record in that table. * Compound key - compound key (also called a composite key or concatenated key) is a key that consists of 2 or more attributes. * Primary key - a primary key is a value that can be used to identify a unique row in a table. Attributes are associated with it. Examples of primary keys are Social Security numbers (associated to a specific person) or ISBNs (associated to a specific book). In the relational model of data, a primary key is a candidate key chosen as the main method of uniquely identifying a tuple in a relation. * Superkey - A superkey is defined in the relational model as a set of attributes of a relation variable (relvar) for which it holds that in all relations assigned to that variable there are no two distinct tuples (rows) that have the same values for the attributes in this set. Equivalently a superkey can also be defined as a set of attributes of a relvar upon which all attributes of the relvar are functionally dependent. * Foreign key - a foreign key (FK) is a field or group of fields in a database record that points to a key field or group of fields forming a key of another database record in some (usually different) table. Usually a foreign key in one table refers to the primary key (PK) of another table. This way references can be made to link information together and it is an essential part of database normalization