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  SQL Server Tips by Burleson

UNIQUE

The UNIQUE constraint determines that no duplicate values are allowed in a column. If a UNIQUE constraint is applied to a composite key, NULL values are accepted and considered the same way as any other values:

A UNIQUE constraint can be clustered as well.

Example: Creating a table with one UNIQUE constraint. Table item_orders2 will have a UNIQUE constraint on column order_id:

CREATE TABLE [dbo].[item_orders2] (
[order_id] [int] CONSTRAINT item_orders2_UNIQUE UNIQUE ,
[item_id] [int] NOT NULL ,
[data] [char] (10) COLLATE SQL_Latin1_General_CP1_CI_AS NULL
) ON [PRIMARY]


Example: Creating a table with one UNIQUE constraint to two columns from an existing table. Table item_orders3 will have a UNIQUE on columns order_id and item_id:

CREATE TABLE [dbo].[item_orders3] (
[order_id] [int] ,
[item_id] [int] NOT NULL ,
[data] [char] (10) COLLATE SQL_Latin1_General_CP1_CI_AS NULL,
CONSTRAINT item_orders3_UNIQUE UNIQUE ([order_id], [item_id])
) ON [PRIMARY]


Example: Adding one UNIQUE constraint to an existing table:

ALTER TABLE [dbo].[item_orders2] ADD CONSTRAINT
UNIQUE_Table2 UNIQUE CLUSTERED
(
order_id
) ON [PRIMARY]

Example: Adding one UNIQUE constraint to two columns from an existing table:

ALTER TABLE [dbo].[item_orders3] ADD CONSTRAINT
UNIQUE_Table3 UNIQUE CLUSTERED
(
order_id,
item_id
) ON [PRIMARY]


The above book excerpt is from:

Super SQL Server Systems
Turbocharge Database Performance with C++ External Procedures

ISBN: 0-9761573-2-2
Joseph Gama, P. J. Naughter

 http://www.rampant-books.com/book_2005_2_sql_server_external_procedures.htm  

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