what is the different between unique+not null & primary key,
Answers were Sorted based on User's Feedback
Answer / sreekumar
In a relation, we can create only one primary key where as we can create multiple unique+not null constrains.
Primary key uses clustered index where are unique +Not null uses non clustered index.
Sreekumar
Is This Answer Correct ? | 18 Yes | 1 No |
Answer / suresh babu
Unique:
Unique is a constraint,which doesn't allow the duplicate
values and allow only one null value.We can create more
number of unique constraint in single table.
Not Null:
Not Null constraint,which doesn't allow the Null values and
allow the duplicate values.
Primary Key:
We can create one primary key for a table,Which doesn't
allow the duplicate and null values,and used for make a
foreign key relation.we can't do this process using above
two constraints.
Is This Answer Correct ? | 4 Yes | 0 No |
Answer / mohan babu
A unique key is nothing but it does not allow any
duplicateds but allows nullvalues
A primary key is nothing but it does not allow duplicates
as well as Null values i.e Not Nulls
Is This Answer Correct ? | 5 Yes | 5 No |
Answer / giri
unique:it does not allow duplicate values but it allow null values.
not null:It does not allow null values but allow duplicate values.
primary:It does not allow both duplicate and null values.
Is This Answer Correct ? | 0 Yes | 0 No |
4. Select sum(A.salary) +sum(B.salary) as TOT_SAL from ( select LEVEL emp_id,level*100 salary,case when mod (level,2)=0then 2 else null end dept_id from dual connect by level<6 )A right outer join (select level emp_id ,level*200 salary ,case when mod (level,3)=0 then 2 else null end dept_id from dual connect by level<6)B On A.dept_id=B.dept_id And A.emp_id=B.emp-id;
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