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Der Constraint UNIQUE versichert uns, dass alle Werte in einem Attribut einzigartig sind. 
CREATE TABLE Persons (
    Id INTEGER NOT NULL,
    LastName VARCHAR(45) NOT NULL,
    FirstName VARCHAR(45),
    Age INTEGER,
    CONSTRAINT UQ_Person_LastName UNIQUE (LastName)
);

ALTER TABLE Persons
ADD CONSTRAINT UQ_Person_LastName UNIQUE (LastName);

ALTER TABLE Persons
DROP INDEX UQ_Person_LastName;