Anil

• Dec 9th, 2006

 

The unique key can take any number of null value.The restriction is for the value which are not null then that value has to be unique.

If we declare that unique key to be primary then they cannot take null value.

It is therefore said that the primary key are unique key which are not null.

SRINIVAS

• Dec 13th, 2006

 

IN UNIQUE NULL VALUES NOT ACCEPTED

datageek

• Dec 13th, 2006

 

Null values are not accepted for keys. (nonulls)Duh!

Kamath

• Dec 13th, 2006

 

There can be only 1 null value for the Uniquekey column.

Motilal

 Profile Answers by Motilal 

• Dec 14th, 2006

 

A Unique key can take any no. of null values.

That is the difference between a unique key and a primary key.

A primary key doesnot allow a null value.

So, here also arises a question that, if a unique key can not take duplicate values, how it accepts multiple null values

The answer is, null values actually correspond to some garbage value at the backend. Which are unique from each other.

Thanks

G.D.S.Prasad

 Profile Answers by G.D.S.Prasad Questions by G.D.S.Prasad

• Dec 19th, 2006

 

Hi,

Thanx Motilal..Very Good explonation

Prasad

amit_vass2000

 Profile Answers by amit_vass2000 

• Dec 19th, 2006

 

plz check it out in any good book. You cannot give wrong information to users. Unique key column can take any number of NULL values because unique key treats every null value different from each other.

Tanushree

• Dec 20th, 2006

 

Unique key accepts any number of NULL values.I have tried it in mysql server.

Shayer

• Dec 20th, 2006

 

In SQL server 2000,if a column is declared as a unique key thenit can accept only one null value.

srinivasan R

• Dec 25th, 2006

 

There can be only one null value in Unique key.

Both Primary and Unique are same, Primary key cannot accept null values, but unique key will accept 1 times null values.

sooraj

• Dec 27th, 2006

 

         the difference between primary key n unique key is that primary key cant accept any null values but unique key can accept one n only one null value.

ritta.singh

 Profile Answers by ritta.singh 

• Dec 30th, 2006

 

A Unique key column can have any number of Null Values, and this is because no 2 null values are equal.

A Null value is a value which does not exists or which exists and is not known or is not applicable in the current context.

neoneo

 Profile Answers by neoneo 

• Jan 3rd, 2007

 

A Unique key column can have any number of Null Values, and this is because no 2 null values are equal. and Column takes the value from Buffer cache.

bibhu

• Jan 17th, 2007

 

By default unique key columns accept null values unless not null constraint is specified.

Pandian S

 Profile Answers by Pandian S Questions by Pandian S

• Mar 5th, 2007

 

Hi,

MSDN Quote : "No two rows in the table are allowed to have the same not null values for the columns in a UNIQUE constraint."

veems26

• Mar 26th, 2007

 

If a column has unique key constraint then, it accepts only one null value.
This is one of the difference between Primary key and Unique key.

mv_sivavardhan

 Profile Answers by mv_sivavardhan 

• Aug 6th, 2008

 

A unique column allows only one NULL value
A Primary key column does not allow single NULL value

rohit1505

 Profile Answers by rohit1505 

• May 21st, 2011

 

Unique key can allow only one null value

Pritesh Modi

 Profile Answers by Pritesh Modi 

• Jun 2nd, 2011

 

there is only one null values accepted by a column if you assign a column as unique key constraint.

thanking you
pritesh

yuvaevergreen

 Profile Answers by yuvaevergreen 

• Jun 14th, 2011

 

In Teradata, only one.