Answer Posted / rajesh

Primary key - Primary key means main key
def:- A primary key is one which uniquely identifies a row
of a table. this key does not allow null values and also
does not allow duplicate values. for ex,
empno empname salary
1 firoz 35000
2 basha 34000
3 chintoo 40000

it will not the values as follows:
1 firoz 35000
1 basha 34000
chintoo 35000

Unique key - single and main key
A unique is one which uniquely identifies a row
of a table, but there is a difference like it will not
allow duplicate values and it will any number of allow
null values(In oracle).
it allows only a single null value(In sql server 2000)

Both will function in a similar way but a slight difference
will be there. So, decalaring it as a primary key is the
best one.

foreign key - a foreign key is one which will refer to a
primary key of another table

for ex,
emp_table dept_table
empno empname salary deptno deptno deptname

In the above relation, deptno is there in emp_table which
is a primary key of dept_table. that means, deptno is
refering the dept_table.

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