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Difference between primary key and unique key ?
 Question Submitted By :: Oracle-General
I also faced this Question!!     Answer Posted By  
# 1
The column holding the primary key constraint cannot accept
null values.whereas colum holding the unique constraint can
accept null values assume that t3 is a table with two
columns t1 having primary key constraint and t2 having
unique constraint if u try to insert null into t2 it will
accept that values whereas column t1 will not accept null
Is This Answer Correct ?    1156 Yes 121 No
# 2
Primary key doesnot allow null value but unique key allows
null value. We can declare only one primary key in a table
but a table can have multiple unique key(column assign).
Is This Answer Correct ?    1010 Yes 126 No
Niranjan Dhungia
# 3
Primery Key = (Unique key - NULL values)

One table may have more than one Unique Key Constraint
columns, but should have only one Primary Key Constraint

Any comment? please shoot a mail.
Is This Answer Correct ?    324 Yes 90 No
# 4
First difference is that primary key doesn't accept null
values whereas unique accepts one or multiple.
second difference is that Clustered index is created on
Primary key constraint and non clustered unique indexes is
created on Unique key constraint.
Is This Answer Correct ?    319 Yes 94 No
Prashant T.
# 5
we can't insert a NULL value into to coloumn holding
primary key constrain.
where as we can insert a NULL value to a unique key feild.
Remember, both keys are used to identify the
rows/tuple/entry of the table uniquely.
so, only once u can enter a NULL value into the unique key
Is This Answer Correct ?    145 Yes 57 No
Austin Johnson
# 6
In the Table a primary key constraint column cannot holding
the null values.whereas unique constraint colum can holding
the null values.
| ID(P) | Name(U) | #ID(P) ->It's a Primary Key
|--------------------| * Its a Not Null
| 1 | Hemendra | * A table Contain 1 Primary
| 2 | Neeraj | Key.
| 3 | PremJe | #Name(U)->It's a Unique Key
|NotNull |canbe Null| * Its can contain Null Value
|--------------------| * A table Contain more
than 1 unique Key.

Please Check the Radio Button..........
******************Gud Luck********************
Is This Answer Correct ?    43 Yes 0 No
Hemendra Singh Chaudhary
# 7
Primary key not accepts to null values in database, but
uniqe key accepts to the null values in database.
Is This Answer Correct ?    76 Yes 35 No
Anil Singh
# 8
the basic difference is that primary key can not have null
value whereas Unique key can have null values.two null
value are not same.One table may have more than one Unique
Key Constraint
columns, but should have only one Primary Key Constraint
Is This Answer Correct ?    87 Yes 51 No
Sunil Upadhyay
# 9
* Only one primary key is allowed per table (any number of
unique keys are allowed).
* Primary key columns automatically become NOT NULL (unique
keys can contain nulls).
* UNIQUE and DISTINCT are synonymous in a SELECT list.
Is This Answer Correct ?    81 Yes 45 No
# 10
7. Primary Key doesnt allow the Null Values. where as in
Unique accept the Null values.
One table may have more than one Unique Key Constraint.
But it sholud have only one Primary Key Constraint column.
Is This Answer Correct ?    76 Yes 47 No
Ark Chowdary

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