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.
The difference is that partial dependency is when a database's attribute is only partially dependent on the primary key. Fully functional dependency is when the attribute is entirely dependent on the key.
Symbols are small, well, symbols that are ON the map. The Key is off to the side, and tells you what the symbols mean.
There are two types of keys in any database; composite and primary keys. Composite key differs from primary key in that it contains more than one column while primary key is composed of only one field and cannot have a null value.
Both third angle and first angle projection display the standard three orthographic views of a part or assembly on a drawing.. The key difference between third angle and first angle is the layout of the part on the sheet.
Yes, multiplicity of values is a key difference between constants and variables. A constant holds a single fixed value that does not change, while a variable can represent multiple values and can change depending on the context or the inputs it receives. This ability to take on different values makes variables essential in mathematical equations and programming, while constants serve as stable references.
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.
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.
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.
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.
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.
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.
don't ask me I'm trying to find out to but i think it has something to do with the government
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.
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.
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.
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.
* 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