database: primary key, Clustered or NonClustered

Question

I am creating a database in SQL server 2008,

CREATE TABLE Users (     U_Id INT NOT NULL     FirstName VARCHAR(50) NOT NULL,     LastName VARCHAR(50) NOT NULL,     Email VARCHAR(200)     Password VARCHAR(50) ) 

I want to make U_Id the primary key. I would like to ask what is the difference between

 CONSTRAINT pk_UserID PRIMARY KEY (U_Id) 

this

 CONSTRAINT pk_UserID PRIMARY KEY CLUSTERED (U_Id) 

and this

CONSTRAINT pk_UserID PRIMARY KEY NONCLUSTERED (U_Id) 

When to use each?

I read some article but it is still unclear to me. Can someone give me a quick explanation?

Answer 1

The following statement:

CONSTRAINT pk_UserID PRIMARY KEY (U_Id) 

Is the same as this one:

CONSTRAINT pk_UserID PRIMARY KEY CLUSTERED (U_Id) 

You can only have the table data physicality ordered by one of the indexes, and by default that index is the one used for the primary key (the primary key unique constraint is always supported by an index).

If you want to leave the order of the table data to be stored according to some other index then you should create the primary key with:

CONSTRAINT pk_UserID PRIMARY KEY NONCLUSTERED (U_Id) 

And then create the clustered index with:

CREATE CLUSTERED INDEX ix_Email ON Users (Email);