A unique constraint is implemented as a unique index.
Examples:
-- Customers must have unique names CREATE UNIQUE INDEX INDEX_Customer_CustomerName ON Customer(CustomerName) -- A non-unique, non-clustered index on a foreign key (to improve performance of JOINs) CREATE INDEX INDEX_Order_CustomerId ON [ORDER](CustomerID) WITH FILLFACTOR = 90 -- A unique, non-cluctered, composite index CREATE UNIQUE INDEX INDEX_Order_Code ON [ORDER](OrderCode, OrderSubcode) ON [PRIMARY]
Define a unique constraint using CREATE TABLE:
CREATE TABLE TestTable ( Id INT IDENTITY(1,1) PRIMARY KEY, -- Named unique constraint TestField CHAR(3) NOT NULL CONSTRAINT INDEX_TestTable_Field UNIQUE NONCLUSTERED, -- Table-level unique constraint CodeID INT NOT NULL DEFAULT 0, SubCodeID INT NOT NULL DEFAULT 0, CONSTRAINT INDEX_TestTable_Code UNIQUE NONCLUSTERED(CodeID, SubCodeID), ) GO
Define a unique constraint using ALTER TABLE:
ALTER TABLE TestTable WITH CHECK ADD CONSTRAINT UN_TestTable_TestField UNIQUE(TestField)