What is the difference b/w Primary Key and Unique Key

Question

I tried to find it out in google but not satisfactory answer is given out there. Can anybody explain the solid difference.

actually if Primary key is used to sele

Answer 1

A primary key does not allow nulls, a unique key will allow one null (on sql server and multiple nulls on Oracle) A table can only have one primary key but many unique keys

use primary keys when you want to set up foreign key relationships

Here is a small example with just one column in each table

--primary key table
CREATE TABLE PrimaryTest (id INT PRIMARY KEY NOT NULL)
GO

-- foreign key table
CREATE TABLE ForeignTest (Pkid INT NOT NULL)
GO


--relationship
ALTER TABLE dbo.ForeignTest ADD CONSTRAINT
    FK_ForeignTest_PrimaryTest FOREIGN KEY
    (
    Pkid
    ) REFERENCES dbo.PrimaryTest
    (
    id
    ) ON UPDATE  NO ACTION 
     ON DELETE  NO ACTION 

GO

insert a row in the primary key table

insert PrimaryTest values(1)

insert a row in the foreign key table with a value that exist in the primary key table

insert ForeignTest values(1)

Now this will fail because value 2 does not exist in the primary key table

insert ForeignTest values(2)

Msg 547, Level 16, State 0, Line 1 The INSERT statement conflicted with the FOREIGN KEY constraint "FK_ForeignTest_PrimaryTest". The conflict occurred in database "aspnetdb", table "dbo.PrimaryTest", column 'id'. The statement has been terminated.