Difference between primary key and unique key ?
- 77 Answers
- 190381 Views
- Accenture, B2B Software Technologies, Cognizant, HP Finsoft, IndiaNIC, Karomi Technology, Keane India Ltd, L&T, Onward eServices, R Systems, Shakti, Techtic Solutions, I also Faced
- E-Mail Answers
Answers were Sorted based on User's Feedback
Answer / umapathy.r
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 ? | 1164 Yes | 121 No |
Answer / niranjan dhungia
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 ? | 1017 Yes | 126 No |
Answer / satya
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
column.
Any comment? please shoot a mail.
| Is This Answer Correct ? | 329 Yes | 90 No |
Answer / prashant t.
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 ? | 320 Yes | 94 No |
Answer / austin johnson
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
feild.
| Is This Answer Correct ? | 146 Yes | 57 No |
Answer / hemendra singh chaudhary
In the Table a primary key constraint column cannot holding
the null values.whereas unique constraint colum can holding
the null values.
Ex.|--------------------|
| 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 ? | 45 Yes | 0 No |
Answer / anil singh
Primary key not accepts to null values in database, but
uniqe key accepts to the null values in database.
| Is This Answer Correct ? | 77 Yes | 35 No |
Answer / sunil upadhyay
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
column.
| Is This Answer Correct ? | 88 Yes | 51 No |
Answer / jeyasree
* 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 ? | 82 Yes | 45 No |
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 ? | 77 Yes | 47 No |
What is meant by a deadlock situation?
Explain the difference between replace() and translate() functions in oracle?
I have a table called 'test' on source and target with same name, structure and data type but in a different column order. How can you setup replication for this table?
Can sub procedure/function be called recursively?
What are the database administrators utilities available?
What are a collation and state the different types of collation sensitivity?
Give the various exception types.
please explain.. DB architecture ...
How to get a list of all background sessions in the database?
Explain the use of ignore option in imp command.
What are the differences between number and binary_float in oracle?
When a form is invoked with call_form, Does oracle forms issues a save point ?
Oracle General 
