- The UNIQUE constraint ensures that all values in a column are different.
- Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns.
- A PRIMARY KEY constraint automatically has a UNIQUE constraint. However, you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.
To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:
CREATE TABLE Persons ( ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int, CONSTRAINT UC_Person UNIQUE (ID,LastName) );
To create a UNIQUE constraint on multiple columns when the table is already created, use the following SQL:
ALTER TABLE Persons ADD CONSTRAINT UC_Person UNIQUE (ID,LastName);
To drop a UNIQUE constraint, use the following SQL:
ALTER TABLE Persons DROP CONSTRAINT UC_Person;