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).

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.

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

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

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.

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.

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

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.