difference between primary key and unique key

Question

I m using mysql database  I have a confusion between primary key and unique key    Please help me where should I create primary and unique key  I mean in which situation we create unique key or primary key

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A primary key must be unique   A unique key does not have to be the primary key - see candidate key   That is  there may be more than one combination of columns on a table that can uniquely identify a row - only one of these can be selected as the primary key  The others  though unique are candidate keys

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For an organization or a business  there are so many physical entities  such as people  resources  machines  etc   and virtual entities  their Tasks  transactions  activities    Typically  business needs to record and process information of those business entities   These business entities are identified within a whole business domain by a Key    As per RDBMS prospective  Key  a k a Candidate Key  is a value or set of values that uniquely identifies an entity    For a DB-Table  there are so many keys are exist and might be eligible for Primary Key   So that all keys  primary key  unique key  etc are collectively called as Candidate Key  However  DBA selected a key from candidate key for searching records is called Primary key   Difference between Primary Key and Unique key  1  Behavior    Primary Key is used to identify a row  record  in a table  whereas Unique-key is to prevent duplicate values in a column  with the exception of a null entry    2  Indexing   By default SQL-engine creates Clustered Index on primary-key if not exists and Non-Clustered Index on Unique-key   3  Nullability  Primary key does not include Null values  whereas Unique-key can   4  Existence  A table can have at most one primary key  but can have multiple Unique-key   5  Modifiability  You can   t change or delete primary values  but Unique-key values can   For more information and Examples   http   dotnetauthorities blogspot in 2013 11 Microsoft-SQL-Server-Training-Online-Learning-Classes-Integrity-Constraints-PrimaryKey-Unique-Key 27 html

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I know this question is several years old but I d like to provide an answer to this explaining why rather than how  Purpose of Primary Key  To identify a row in a database uniquely     A row represents a single instance of the entity type modeled by the table  A primary key enforces integrity of an entity  AKA Entity Integrity  Primary Key would be a clustered index i e  it defines the order in which data is physically stored in a table   Purpose of Unique Key  Ok  with the Primary Key we have a way to uniquely identify a row  But I have a business need such that  another column a set of columns should have unique values  Well  technically  given that this column s  is  unique  it can be a candidate to enforce entity integrity  But for all we know  this column can contain data originating from an external organization that I may have a doubt about being unique  I may not trust it to provide entity integrity  I just make it a unique key to fulfill my business requirement    There you go

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If your Database design is such that their is no need of foreign key  then you can go with Unique key  but remember unique key allow single null value      If you database demand foreign key then you leave with no choice you have to go with primary key   To see the difference between unique and primary key visit here

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A Primary key is a unique key   Each table must have at most ONE primary key but it can have multiple unique key  A primary key is used to uniquely identify a table row  A primary key cannot be NULL since NULL is not a value

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difference between Primary Key and Unique Key  Both Primary key and Unique Key are used to uniquely define of a row in a table   Primary Key creates a clustered index of the column whereas a Unique creates an unclustered index of the column    A Primary Key doesn   t allow NULL value  however a Unique Key does allow one NULL value

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Simply Primary Key is a unique and can t be null  unique can be null and may not be unique

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Primary Keys  The main purpose of the primary key is to provide a means to identify each record in the table   The primary key provides a means to identity the row  using data within the row   A primary key can be based on one or more columns  such as first and last name  however  in many designs  the primary key is an auto-generated number from an identity column   A primary key has the following characteristics    There can only be one primary key for a table  The primary key consists of one or more columns  The primary key enforces the entity integrity of the table  All columns defined must be defined as NOT NULL  The primary key uniquely identifies a row  Primary keys result in CLUSTERED unique indexes by default    Unique Keys  A unique key is also called a unique constraint   A unique constraint can be used to ensure rows are unique within the database     Don   t we already do that with the primary key   Yep  we do  but a table may have several sets of columns which you want unique   In SQL Server the unique key has the following characteristics    There can be multiple unique keys defined on a table  Unique Keys result in NONCLUSTERED Unique Indexes by default  One or more columns make up a unique key  Column may be NULL  but on one NULL per column is allowed  A unique constraint can be referenced by a Foreign Key Constraint    source   here

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Think the table name is employe  Primary key Primary key can not accept null values  primary key enforces uniqueness of a  column  We can have only one Primary key in a table  Unique key Unique key can accept null values  unique key also enforces uniqueness of a column you can think if unique key contains null values then why it can be unique   yes  though it can accept null values it enforces uniqueness of a column  just have a look on the picture here Emp ID is primary and Citizen ID is unique  Hope you understand  We can use multiple unique key in a table

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Primary Key    There can only be one primary key in a table In some DBMS it cannot be NULL - e g  MySQL adds NOT NULL Primary Key is a unique key identifier of the record   Unique Key    Can be more than one unique key in one table Unique key can have NULL values It can be a candidate key Unique key can be NULL   multiple rows can have NULL values and therefore may not be considered  unique

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Unique Key  UK   It s a  column or a group of columns that can identify a uniqueness in a row    Primary Key  PK   It s also a  column or group of columns that can identify a uniqueness in a row   So the Primary key is just another name for unique key  but the default implementation in SQL Server is different for Primary and Unique Key    By Default      PK creates a Clustered index and UK creates a Non Clustered Index   PK is not null  but UK allows nulls  Note  By Default  There can only be one and only one PK on a table  but there can be multiple UK s You can override the default implementation depending upon your need     It really depends what is your aim when deciding whether to create a UK or PK  It follows an analogy like   If there is a team of three people  so all of them are peers  but there will be one of them who will be a pair of peers  PK and UK has similar relation    I would suggest reading this article  The example given by the author may not seem suitable  but try to get an overall idea    http   tsqltips blogspot com 2012 06 difference-between-unique-key-and html

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A primary key   s main features are   It must contain a unique value for each row of data  It cannot contain null values  Only one Primary key in a table   A Unique key   s main features are   It can also contain a unique value for each row of data   It can also contain null values   Multiple Unique keys in a table

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A primary key has the semantic of identifying the row of a database  Therefore there can be only one primary key for a given table  while there can be many unique keys   Also for the same reason a primary key cannot be NULL  at least in Oracle  not sure about other databases   Since it identifies the row it should never ever change  Changing primary keys are bound to cause serious pain and probably eternal damnation   Therefor in most cases you want some artificial id for primary key which isn t used for anything but identifying single rows in the table   Unique keys on the other hand may change as much as you want

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Difference between Primary Key and Unique Key  ----------------------------------------- -----------------------------------------------                   Primary Key                                   Unique Key                    ----------------------------------------- -----------------------------------------------    Primary Key can t accept null values      Unique key can accept only one null value        ----------------------------------------- -----------------------------------------------    By default  Primary key is clustered      By default  Unique key is a unique                index and data in the database table is   non-clustered index                               physically organized in the sequence of                                                     clustered index                                                                            ----------------------------------------- -----------------------------------------------    We can have only one Primary key in a     We can have more than one unique key in a         table                                     table                                            ----------------------------------------- -----------------------------------------------    Primary key can be made foreign key       In SQL Server  Unique key can be made foreign     into another table                        key into another table                           ----------------------------------------- -----------------------------------------------    You can find detailed information from  http   www dotnet-tricks com Tutorial sqlserver V2bS260912-Difference-between-Primary-Key-and-Unique-Key html

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Unique key  -                  It should be used when you have to give unique value In the case of                  unique key it means null values are also allowed Unique keys are those                  keys which are unique and non similar in that column like for example                  your pet name it can be nothing like null and if you are asking in   context of database then it must be noted that every null is different from another null in the database EXCEPT-SQL Server where null null is true        primary key  -                   It should be used when you have to give uniquely identify a row primary is key which unique for every row in a database constraint is that it doesn t allow null in it so  you might have seen that the database have a column which is auto increment and it is the primary key of the table  plus it can be used as a foreign key in another table example can be orderId on a order Table billId in a bill Table       now coming back to situation when to use it -   1  primary key in the column which can  not be null in the table and you are using as foreign key in another table for creating relationship   2  unique key in table where it  doesn t affect in table or in the whole database whether you take the  null for the particular column like snacks in the restaurant it is  possible you don t take snacks in a restaurant

[mysql] difference between primary key and unique key

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