What is the difference between a candidate key and a primary key

Question

Is it that a primary key is the selected candidate key chosen for a given table

User · Answer

There is no difference  A primary key is a candidate key  By convention one candidate key in a relation is usually chosen to be the  primary  one but the choice is essentially arbitrary and a matter of convenience for database users designers developers  It doesn t make a  primary  key fundamentally any different to any other candidate key

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John Woo s answer is correct  as far as it goes   Here are a few additional points   A primary key is always one of the candidate keys  Fairly often  it s the only candidate   A table with no candidate keys does not represent a relation   If you re using the relational model to help you build a good database  then every table you design will have at least one candidate key   The relational model would be complete without the concept of primary key   It wasn t in the original presentation of the relational model   As a practical matter  the use of foreign key references without a declared primary key leads to a mess   It could be a logically correct mess  but it s a mess nonetheless   Declaring a primary key lets the DBMS help you enforce the data rules   Most of the time  having the DBMS help you enforce the data rules is a good thing  and well worth the cost   Some database designers and some users have some mental confusion about whether the primary key identifies a row  record  in a table or an instance of an entity in the subject matter that the table represents   In an ideal world  it s supposed to do both  and there should be a one-for-one correspondence between rows in an entity table and instances of the corresponding entity     In the real world  things get screwed up   Somebody enters the same new employee twice  and the employee ends up with two ids    Somebody gets hired  but the data entry slips through the cracks in some manual process  and the employee doesn t get an id  until the omission is corrected   A database that does not collapse the first time things get screwed up is more robust than one that does

User · Answer

A table can have so many column which can uniquely identify a row  This columns are referred as candidate keys   but primary key should be one of them because one primary key is enough for a table  So selection of primary key is important among so many candidate key  Thats the main difference

User · Answer

Primary key -  Any column or set of columns that can uniquely identify a record in the table is a primary key   There can be only one Primary key in the table  and  the candidate key-  the same as Primary key but the Primary Key chosen by DB administrator s prospective for example the primary  key the least candidate key in size

User · Answer

Think of a table of vehicles with an integer Primary Key     The registration number would be a candidate key   In the real world registration numbers are subject change so it depends somewhat on the circumstances what might qualify as a candidate key

User · Answer

Candidate Key     A Candidate Key can be any column or a combination of columns that can qualify as unique key in database  There can be multiple Candidate Keys in one table  Each Candidate Key can qualify as Primary Key   Primary Key     A Primary Key is a column or a combination of columns that uniquely identify a record  Only one Candidate Key can be Primary Key   More on this link with example

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If superkey is a big set than candidate key is some smaller set inside big set and primary key any one element one at a time or for a table  in candidate key set

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A Primary key is a special kind of index in that   there can be only one  it cannot be nullable it must be unique    Candidate keys are selected from the set of super keys  the only thing we take care while selecting the candidate key is  It should not have any redundant attribute    Example of an Employee table  Employee   Employee ID  FullName  SSN  DeptID     Candidate Key  are individual columns in a table that qualifies for the uniqueness of all the rows  Here in Employee table EmployeeID  amp  SSN are Candidate keys  Primary Key  are the columns you choose to maintain uniqueness in a table  Here in Employee table  you can choose either EmployeeID or SSN columns  EmployeeID is a preferable choice  as SSN is a secure value  Alternate Key  Candidate column other the Primary column  like if EmployeeID is PK then SSN would be the Alternate key  Super Key  If you add any other column attribute to a Primary Key then it becomes a super key  like EmployeeID   FullName  is a Super Key  Composite Key  If a table does not have a single column that qualifies for a Candidate key  then you have to select 2 or more columns to make a row unique  Like if there is no EmployeeID or SSN columns  then you can make FullName   DateOfBirth as Composite primary Key  But still  there can be a narrow chance of duplicate row

User · Answer

A primary key is a column  or columns  in a table that uniquely identifies the rows in that table   CUSTOMERS   CustomerNo  FirstName   LastName 1   Sally   Thompson 2   Sally   Henderson 3   Harry   Henderson 4   Sandra  Wellington   For example  in the table above  CustomerNo is the primary key   The values placed in primary key columns must be unique for each row  no duplicates can be tolerated  In addition  nulls are not allowed in primary key columns   So  having told you that it is possible to use one or more columns as a primary key  how do you decide which columns  and how many  to choose   Well there are times when it is advisable or essential to use multiple columns  However  if you cannot see an immediate reason to use multiple columns  then use one  This isn t an absolute rule  it is simply advice  However  primary keys made up of single columns are generally easier to maintain and faster in operation  This means that if you query the database  you will usually get the answer back faster if the tables have single column primary keys   Next question     which column should you pick  The easiest way to choose a column as a primary key  and a method that is reasonably commonly employed  is to get the database itself to automatically allocate a unique number to each row   In a table of employees  clearly any column like FirstName is a poor choice since you cannot control employee s first names  Often there is only one choice for the primary key  as in the case above  However  if there is more than one  these can be described as  candidate keys      the name reflects that they are candidates for the responsible job of primary key

User · Answer

First you have to know what is a determinant  the determinant is an attribute that used to determine another attribute in the same table  SO the determinant must be a candidate key  And you can have more than one determinant  But primary key is used to determine the whole record and you can have only one primary key  Both primary and candidate key can consist of one or more attributes

User · Answer

Primary key -  Any column or set of columns that can uniquely identify a record in the table is a primary key   There can be only one Primary key in the table   Candidate key -  Any column or set of columns that are candidate to become primary key are Candidate key   There can be one or more candidate key s  in the table  if there is only one candidate key  it can be chosen as Primary key

[sql] What is the difference between a candidate key and a primary key?

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