Just as simple as this...

-- Adding constraint to an existing table

ALTER TABLE SomeTable WITH CHECK ADD CONSTRAINT [UK_SomeTable_SomeCompositeKey] UNIQUE( KeyColumn1, KeyColumn2 )

-- Adding constraint when creating new table

CREATE TABLE SomeTable

(

KeyColumn1 INT NOT NULL,

KeyColumn2 INT NOT NULL,

CONSTRAINT [UK_SomeTable_SomeCompositeKey] UNIQUE

(

KeyColumn1,

KeyColumn2

)

)

Edit:

1. The ADDADD thing is the mis-interpretation of SSC, just use single ADD in the query

2. Added an example of adding constraint in a CREATE statement.