Answer / ashwin

you can insert as many values as u want, in uniquelly
declared column

some people r saying that if u insert more than one null
value in unique column of relation then null is duplicated
which violets unique key property..........well this saying
is totally rubbish......are pehle insert karke dekho phir
bolo as simple as that...

Answer / shailendra chauhan

Yes, We can insert more the one NULL with unique key.
cos, Neither null is equal to null Nor null is not equal to
null.

Answer / christina

Yes, We can use more then one null value in unique key ,
because "null" means undefined or unknown so we can add any
number of null values in unique key...
This is the major difference between primary nd unique key,
Primary key doesn't allow null values to be entered into
the columns marked as priamry key and Unique allow us to
enter null values...............

Answer / sam

Yes ,we can insert more null value in a table having unique
key,Bcoz one null vale is different frm another null value.

Answer / arika

We can insert duplicate nul values in palce of using unique
key constrint.Because all nulls are not equal.


ex:-
select * from emp where comm=null;
explenation;
here equal operator not support null value because
all nulls are not equal
ex:-
select * from emp where comm is null;
explenation:
'is' operator suport null

Answer / praveen hr

UNIQUE Constraints

You can use UNIQUE constraints to make sure that no
duplicate values are entered in specific columns that do
not participate in a primary key. Although both a UNIQUE
constraint and a PRIMARY KEY constraint enforce uniqueness,
use a UNIQUE constraint instead of a PRIMARY KEY constraint
when you want to enforce the uniqueness of a column, or
combination of columns, that is not the primary key.

Multiple UNIQUE constraints can be defined on a table,
whereas only one PRIMARY KEY constraint can be defined on a
table.

Also, unlike PRIMARY KEY constraints, UNIQUE constraints
allow for the value NULL. However, as with any value
participating in a UNIQUE constraint, only one null value
is allowed per column.

A UNIQUE constraint can be referenced by a FOREIGN KEY
constraint.

Link from MSDN http://msdn.microsoft.com/en-
us/library/ms191166.aspx

Answer / a

we have to use computed column to insert multiple null values.

Answer / raman

Primary Key is a combination of Unique and NOT NULL
Constraints so it can’t have duplicate values or any NUll

Whereas for Oracle UNIQUE Key can have any number of NULL
but for SQL Server It can have only one NULL

Answer / hassan khan

A unique key constraint does not imply the NOT NULL
constraint in practice. Because NULL is not an actual value
(it represents the lack of a value), when two rows are
compared, and both rows have NULL in a column, the column
values are not considered to be equal. Thus, in order for a
unique key to uniquely identify each row in a table, NULL
values must not be used. According to the SQL standard and
Relational Model theory, a unique key (unique constraint)
should accept NULL in several rows/tuples — however not all
RDBMS implement this feature correctly.

Answer / sabariesh thavamani

i worked out the coding in the above using oracle 9 i, but
i could not get the correct answer what he told.. try it

SQL> create table sab(name varchar2(10) unique);

Table created.

SQL> insert into sab values('&sab');
Enter value for sab: null
old 1: insert into sab values('&sab')
new 1: insert into sab values('null')

1 row created.

SQL> /
Enter value for sab: null
old 1: insert into sab values('&sab')
new 1: insert into sab values('null')
insert into sab values('null')
*
ERROR at line 1:
ORA-00001: unique constraint (LPUSER18.SYS_C0038068)
violated


SQL> /
Enter value for sab: sabari
old 1: insert into sab values('&sab')
new 1: insert into sab values('sabari')

1 row created.

SQL> /
Enter value for sab: null
old 1: insert into sab values('&sab')
new 1: insert into sab values('null')
insert into sab values('null')
*
ERROR at line 1:
ORA-00001: unique constraint (LPUSER18.SYS_C0038068)
violated


SQL> select * from sab
2 ;

NAME
----------
null
sabari
try it....

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