What are the differences between a superkey and a candidate key

Question

What are the differences between a super key and a candidate key  I have already referred to wiki dotnet spider and also Database Concepts 4th edition book  But I am unable to understand the concept  Can anyone please explain it with proper example

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In nutshell  CANDIDATE KEY is a minimal SUPER KEY   Where Super key is the combination of columns or attributes  that uniquely identify any record or tuple  in a relation table  in RDBMS     For instance  consider the following dependencies in a table having columns A  B  C  and D  Giving this table just for a quick example so not covering all dependencies that R could have    Attribute set  Determinant ---Can Identify---  Dependent   A-----  AD  B-----  ABCD  C-----  CD  AC----- ACD  AB----- ABCD  ABC----- ABCD  BCD----- ABCD    Now  B  AB  ABC  BCD identifies all columns so those four qualify for the super key   But  B AB  B ABC  B BCD hence AB  ABC  and BCD disqualified for CANDIDATE KEY as their subsets could identify the relation  so they aren t minimal and hence only B is the candidate key  not the others   One more thing Primary key is any one among the candidate keys   Thanks for asking

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Candidate key is a super key from which you cannot remove any fields   For instance  a software release can be identified either by major minor version  or by the build date  we assume nightly builds    Storing date in three fields is not a good idea of course  but let s pretend it is for demonstration purposes   year  month date  major  minor 2008  01    13     0      1 2008  04    23     0      2 2009  11    05     1      0 2010  04    05     1      1   So  year  major  minor  or  year  month  date  major  are super keys  since they are unique  but not candidate keys  since you can remove year or major and the remaining set of columns will still be a super key    year  month  date  and  major  minor  are candidate keys  since you cannot remove any of the fields from them without breaking uniqueness

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One candidate key is chosen as the primary key  Other candidate keys are called alternate keys

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Super key    super key is a set of atttibutes in a relation table  which can define every tupple in the relation table  uniquely   Candidate key   we can say minimal super key is candidate key     Candidate is the smallest sub set of super key  And can uniquely define each and every tupple

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A super key of an entity set is a set of one or more attributes whose values uniquely determine each entity   A candidate key of an entity set is a minimal super key   Let s go on with customer  loan and borrower sets that you can find an image from the link    1   Rectangles represent entity sets    1 Diamonds represent relationship set    1 Elipses represent attributes    1 Underline indicates Primary Keys    1   customer id is the candidate key of the customer set  loan number is the candidate key of the loan set   Although several candidate keys may exist  one of the candidate keys is selected to be the primary key   Borrower set is formed customer id and loan number as a relationship set

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Superkey  A set of attributes or combination of attributes which uniquely identify the tuple in a given relation   Superkey have two properties uniqueness and reducible set  Candidate key  Minimal set of superkey which have following two properties  uniqueness and irreducible set or attribute

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Super Key  A superkey is any set of attributes for which the values are guaranteed to be unique for all possible set of tuples in a table at all time   Candidate Key  A candidate key is a  minimal  super key meaning the smallest subset of superkey attribute which is unique

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A Super key is a set or one of more columns  to uniquely identify rows in a table    Candidate keys are selected from the set of super keys  the only thing we take care while selecting candidate key is  It should not have any redundant attribute  That   s the reason they are also termed as minimal super key   In Employee table there are Three Columns   Emp Code Emp Number Emp Name  Super keys    All of the following sets are able to uniquely identify rows of the employee table    Emp Code   Emp Number   Emp Code  Emp Number   Emp Code  Emp Name   Emp Code  Emp Number  Emp Name   Emp Number  Emp Name    Candidate Keys   As I stated above  they are the minimal super keys with no redundant attributes    Emp Code   Emp Number    Primary key   Primary key is being selected from the sets of candidate keys by database designer  So Either  Emp Code  or  Emp Number  can be the primary key

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Super key is the combination of fields by which the row is uniquely identified and the candidate key is the minimal super key

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A super key is any combination of columns that uniquely identifies a row in a table  A candidate key is a super key which cannot have any columns removed from it without losing the unique identification property   This property is sometimes known as minimality or  better  irreducibility   A super key   a primary key in general  The primary key is simply a candidate key chosen to be the main key  However  in dependency theory  candidate keys are important and the primary key is not more important than any of the other candidate keys  Non-primary candidate keys are also known as alternative keys    Consider this table of Elements   CREATE TABLE elements       atomic number   INTEGER NOT NULL PRIMARY KEY                     CHECK  atomic number  gt  0 AND atomic number  lt  120       symbol          CHAR 3  NOT NULL UNIQUE      name            CHAR 20  NOT NULL UNIQUE      atomic weight   DECIMAL 8 4  NOT NULL      period          SMALLINT NOT NULL                     CHECK  period BETWEEN 1 AND 7       group           CHAR 2  NOT NULL                     --  L  for Lanthanoids   A  for Actinoids                     CHECK  group IN   1    2    L    A    3    4    5    6                                         7    8    9    10    11    12    13                                         14    15    16    17    18         stable          CHAR 1  DEFAULT  Y  NOT NULL                     CHECK  stable IN   Y    N         It has three unique identifiers - atomic number  element name  and symbol   Each of these  therefore  is a candidate key   Further  unless you are dealing with a table that can only ever hold one row of data  in which case the empty set  of columns  is a candidate key   you cannot have a smaller-than-one-column candidate key  so the candidate keys are irreducible   Consider a key made up of   atomic number  element name  symbol     If you supply a consistent set of values for these three fields  say   6  Carbon  C     then you uniquely identify the entry for an element - Carbon   However  this is very much a super key that is not a candidate key because it is not irreducible  you can eliminate any two of the three fields without losing the unique identification property   As another example  consider a key made up of   atomic number  period  group     Again  this is a unique identifier for a row    6  2  14   identifies Carbon  again    If it were not for the Lanthanoids and Actinoids  then the combination of   period  group   would be unique  but because of them  it is not   However  as before  atomic number on its own is sufficient to uniquely identify an element  so this is a super key and not a candidate key

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Basically  a Candidate Key is a Super Key from which no more Attribute can be pruned   A Super Key identifies uniquely rows tuples in a table relation of a database  It is composed by a set of attributes that combined can assume values unique over the rows tuples of a table relation  A Candidate Key is built by a Super Key  iteratively removing pruning non-key attributes  keeping an invariant  the newly created Key still need to uniquely identifies the rows tuples   A Candidate Key might be seen as a minimal Super Key  in terms of attributes   Candidate Keys can be used to reference uniquely rows tuples but from the RDBMS engine perspective the burden to maintain indexes on them is far heavier

[database] What are the differences between a superkey and a candidate key?

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