What is difference between "Primary key" and "Unique key"?

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  • Mar 18th, 2006

A unique key has the property that no two values in the range are the same.  A primary key is chosen from among all the unique keys of a table to participate in referential integrity constraints. Primary keys are unique by definition.

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  • Mar 20th, 2006

Primary key and unique are Entity integrity constraints

Primary key allows each row in a table to be uniquely identified and ensures that no duplicate rows exist and no null values are entered.

Unique key constraint is used to prevent the duplication of key values within the rows of a table and allow null values. (In oracle, one null is not equal to another null).

Rana Irfan

  • Mar 24th, 2006

Primary key can't be null but unique key can be null.

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  • Mar 24th, 2006

sorry rana, unique constraints do not include null.  Null is not a value it's a state where the value is unknown. In DB2 a unique constraint MUST include the NOT NULL clause.

Even if allowed, the index used to guarantee uniqueness could at most have one entry for NULL. This would assume that NULL was treated as a value. DB2 has a special property for an index namely: UNIQUE WHERE NOT NULL, which allows multiple instances of NULL but forces unique values. This is good for M:1 relations where the many side is optional in the relationship. DB2's index is a very special case. Don't go looking for it in other RDBMS products.

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pramod kumar

  • Apr 6th, 2006

1)unique key can be null but primariy key cant be null.

2)primariy key can be refrenced to other table as FK.


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pramod kumar

  • Apr 6th, 2006

1)unique key can be null but primariy key cant be null.

2)primariy key can be refrenced to other table as FK.

3)we can have

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pramod kumar

  • Apr 6th, 2006

1)unique key can be null but primariy key cant be null.

2)primariy key can be refrenced to other table as FK.

3)we can have multiple unique key in a table but PK is one and only one.

4)PK in itself is unique key.


  • Apr 6th, 2006

UNIQUE KEY accepts null value whereas primary key does not accept null value and

a table can have more then one unique key column ,but a table can have only one primary key column

primary key creates clustered index ,but unique key creates non clustered index.

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  • Apr 20th, 2006

Primary which allows only unique+not null values

where has unique allow unique and null values because  a null value is not equal to a null value          

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Giridhar Ks

  • May 7th, 2006

Primary Key:- Primary Key is an attribute or a set of attributes in a table which uniquely identifies a record (row) in a table and no two records (rows) in the table can have the same values for all the columns comprising the primary key.

Unique Key:- A unique key is a key which stores unique values (no duplicates) for that particular column.

Difference between Primary Key & Unique Key:-

1. A Unique key can store a null value and primary key cannot store any null value.

2. A primary key can be references to another table as a Foreign Key.

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Vinay Kumar

  • May 29th, 2007

one more difference is:-

Primary key creates the Clustered index, but unique key creates the Non clustered index.

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Ajitabh Kumar

  • Jun 8th, 2007

Primary key accepts only non duplicate value and not accept null value and only one primary key may have in table

Unique key can have more than one null value , there may be more than one unique key in a table

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Primary key: -
                     1) Can only one in a table
                     2) We can't insert duplicate value or same value
                     3) Can use as a foreighen key for other table
                     4) It never allow Null values.
Unique key: -
                     1) There can be more than one unique key in one table
                     2) Unique key can have Null Values
                     3) It can't be a candidate key

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  • Jun 9th, 2008

Primary can't be null but unique key allows one null value..
PRIMARY KEY:In a table we can create only one primarykey 
UNIQUE KEY: we can create no. of unique keys in a table..

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  • Jun 30th, 2009

Following are the differences between Primary and Unique constraints:

1) The columns that are primary keys cannot contain NULL values as primary key is meant to uniquely identify a record in a table. The columns that make a unique key can be nullable as unique key just restricts a column not to hold duplicates .

2) There can be only one primary key per table while there can be N number of unique keys defined in a table.

3) By default a clustered index is created on a primary key while a non clustered index is created on a unique key.


  • Jul 1st, 2009

Primary Key is Key which don't allows any duplicate values or null values to be inserted. Whereas, Unique key also don't allows duplicate values to be inserted but it allows null values to be inserted because null values cannot be compared to anything.

Dharmendra Jaiswal

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  • Feb 17th, 2010

The main difference of primary key and the unique key is:

Primary key does not allow null values as well as duplicate values.

But incase of unique key it does not allow duplicate values but it allows null values.

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  • Mar 2nd, 2010

A primary key is both unique and not null.  There can only be one primary key per row.  It is what defines the row.  The primary key is the 'name' of the row.

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  • Jul 11th, 2010

Primary key is used to identify row which is unique and not null
Unique key is used to identify distinct row, it can contian null value

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  • Jul 12th, 2010

Primay Key: Unique field in the table which cannot be duplicated and cannot be NULL.
Unique Key: Uniqueu field in the table but can be NULL

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  • Dec 30th, 2010

The Difference between Primary Key and Unique is as follows :

1. Primary Key does not allow null value but Unique key allow.

2. In a Table There will be only one primary key but Unique key cam be more than  one.

3. In case of creating a master - detail relation between two tables
If You are referencing Primary key in another table then no need to specify column name but in case of unique you must have to specify column name.

4. Primary Key creates clustered index where as unique creates non clustered index.

Example for Primary Key:-



Example for Unique Key :-



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