constraint expression for unique keys

[HandersonVA]

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?
thanks

[Alexander Kuznetsov]

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

"

[HandersonVA]

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

"

[Alexander Kuznetsov]

SQL Server implicitly creates an index to implement a uinque
constraint. In this case NONCLUSTERED means the index will be
non-clustered.

[Erland Sommarskog]

HandersonVA (ha*********@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, es****@sommarskog.se

Books Online for SQL Server 2005 at
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[--CELKO--]

>> 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 :)