alok

• Aug 1st, 2007

 

Unique+Not null can be defined in a table more than once.
Primary key has an index created for it implicietly.

prasads_delight1

 Profile Answers by prasads_delight1 Questions by prasads_delight1

• Sep 18th, 2007

 

               We can give the not null, unique constrints to the fields in the table. Actually the combination of the properties of these two constrins are present in the primary key constraint. If the primarykey constraint is divided into two parts the two parts will be UNIQE and NOT NULL.

rajakumar_na

 Profile Answers by rajakumar_na 

• Nov 19th, 2007

 

NOT NULL: It allows duplicates, but doesn't allow null values
UNIQUE: It allows null values, but doesn't allow duplicates
PRIMARY KEY: It doesn't allows duplicates and null values

Unique index wil be created automatically for PRIMARY KEY, and UNIQUE constraints, but index wil not created implicitly 4  NOT NULL constraint.

There only one PRIMARY KEY wil be alowed 4 a table... we can create any number of NOT NULL+UNIQUE constraint 4 a table.

sampra

 Profile Answers by sampra 

• Mar 6th, 2012

 

primary key cant be null where as unique key can be null

aytasirs

 Profile Answers by aytasirs 

• Nov 15th, 2012

 

We can create only 1 primary constraint per a table, But Can create more than 1 unique+Not null Constraints per a table.

EHSAN REHMANI

• Apr 27th, 2013

 

There is only one primary key per table but we can use unique and not null constraints more than one column in a table. Primary key having both functionality as it can not be null but unique.

aytasirs

• Apr 29th, 2013

 

Yes We can. If we use the combination "unique+not null" the referential integrity is not maintained and that is not the case with primary key.