text: SET @firstname = 'John', @lastname = 'Doe', @email = 'john@example.com';
  
  INSERT INTO Teste (firstname, lastname, email)
  SELECT * FROM (SELECT @firstname, @lastname, @email) AS tmp
  WHERE NOT EXISTS (
      SELECT firstname, lastname, email FROM Teste WHERE firstname = @firstname AND lastname = @lastname AND email = @email
  ) LIMIT 1;