Answer Posted / sandeep beniwal (master)

Primary key:- Primary key means main key. A primary key is
one which uniquely identifies of a table. Prrimay key does
not allow null values and duplicate values.
e.g:
empno empname status
10 dipender active
20 nisha inactive
30 vikas active
40 naveen active
Pimary key not like this:
10 dipender active
20 nisha inactive
30 vikas active
naveen active ........ (duplicate value)

Unique key:-A unique is one which uniquely identifies a row
of a table, but there is a difference like it will allow
only one duplicate values (In sql).

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 and helps making link

e.g
employee_table user_table
empID empname status userID userID empname
10 dipender active 1 1 dipender
20 nisha inactive 2 2 nisha
30 vikas active 3 3 vikas
40 naveen active 4 4 naveen

In the above relation, userID is there in employee_table
which
is a primary key of user_table. that means, userID is
refering the user_table.

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