constraint expression for unique keys

if i have a table which defines a rule as "combination of two field
must be unique", how can I write this in a constraint expression
section?
i started learning more about ms sql side to handle all the necessary
rules in back-end instead of front-end.
also any good learning links, references, or book recommandations?
thanksan excerpt from BOL:

"C. Using UNIQUE constraints
UNIQUE constraints are used to enforce uniqueness on nonprimary key
columns. The following example enforces a restriction that the Name
column of the Product table must be unique.

Copy Code
Name nvarchar(100) NOT NULL
UNIQUE NONCLUSTERED

"|||can you explain what "NONCLUSTERED" is doing there? is that for
non-relation to a field in other table?

Alexander Kuznetsov wrote:
> an excerpt from BOL:
> "C. Using UNIQUE constraints
> UNIQUE constraints are used to enforce uniqueness on nonprimary key
> columns. The following example enforces a restriction that the Name
> column of the Product table must be unique.
> Copy Code
> Name nvarchar(100) NOT NULL
> UNIQUE NONCLUSTERED
> "|||SQL Server implicitly creates an index to implement a uinque
constraint. In this case NONCLUSTERED means the index will be
non-clustered.|||HandersonVA (handersonva@.hotmail.com) writes:
> if i have a table which defines a rule as "combination of two field
> must be unique", how can I write this in a constraint expression
> section?

CONSTRAINT u_tbl UNIQUE (col1, col2)

--
Erland Sommarskog, SQL Server MVP, esquel@.sommarskog.se

Books Online for SQL Server 2005 at
http://www.microsoft.com/technet/pr...oads/books.mspx
Books Online for SQL Server 2000 at
http://www.microsoft.com/sql/prodin...ions/books.mspx|||>> if i have a table which defines a rule as "combination of two field must be unique", how can I write this in a constraint expression section? <<

CONSTRAINT unique_location UNIQUE (x, y)
>> any good learning links, references, or book recommandations? <<

I recommend buying all of my books :)