In primary key  not allowed any null value while in unique 
key one null value allowed.

By default Primary Key Create Cluster index while unique 
ker create non cluster index

only 1 p.K. have per table

u can have multipale Unique key per table

Primary key is a combination of unique+not null+cluster 
index and it will not allow any duplicate values, null 
values, and it's data will be arranged in ascending order 
due to clustered index.  Only one primary key is allowed 
per table.

Unique key is a key which will not accept any duplicate 
values, but it will accept one null value.  We can place 
any number of unique keys per table.

PRIMARY KEY AND UNIQUE KEY

   * Both Enforces Uniqueness
   * Primary key  Creates clustered index
     unique key creates non clustered index 
   * primay key does not allows null value
     unique key allows 1 null value

The difference is primary key does not allow any one null 
value,but unique allow only one null value,because it 
consider null as one value

There are difference between Primary key and unique key.

1. Primary key allow not null value where as unique key 
allow only one null value.

2. Primary key  Creates clustered index where as unique key 
creates non clustered index.
 
3. Primary key is only one in a table where as unique key 
is multiple in a table.

4. Primary key is not accept duplicate value where as 
Unique key is also not accept any duplicate values, but it 
will accept one null value.

HI ALL

IN CASE OF INSERTING REPETED NULL IN UNIQUE KEY COLOUMN ,
IF IT IS COMPOSITE KEY IS POSSIBLE. . . ANY LOGICAL RESON?

Eg:
CREATE TABLE TRY(ROLL NUMBER,LIB_CARD_NO NUMBER,
               CONSTRAINT CON_UNQ UNIQUE(ROLL,LIB_CARD_NO));

INSERT INTO TRY VALUES(NULL,NULL); 
--> 1 ROW CREATED
INSERT INTO TRY VALUES(NULL,NULL); 
--> 1 ROW CREATED -- REPETED NULL ARE ALLOWED!!!!