Answer Posted / raviraj bhardwaj

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 priya 35000
2 preeti 34000
3 niharika 40000

it will not the values as follows:
1 priya 35000
1 preeti 34000
niharika 35000


unique key~ unique key creates non clustered index by
default.it allows "one null value".

forigen key~
when the primary key of one table injected in other table to
make a relationship then the other key of the other table is
called foreign key.
for ex-
Dept table employs
deptno deptmame deptloc eid ename eadd deptno

in it deptno inter relate bothe the tables so that deptno
works here as a forigen kye..

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