Unique constraints are like alternative primary key constraints. A unique constraint defines a column, or series of columns, that must be unique in value. You can have a number of unique constraints defined and the columns can have NULL values in them, unlike a column that belongs to a primary key constraint. If you need to add unique key constraints to a table after the fact, simply use the alter table command.
Syntax:
CREATE TABLE table_name
(
column1 datatype Unique,
column1 datatype,
column1 datatype
);
