Oracle: How to avoid duplicate rows in table while inserting and updating data

There are several ways to avoid storing duplicate rows in an Oracle database when inserting and updating data. In the examples I use a table with two columns that should be unique:

1. Unique keys

ALTER TABLE tableName
    ADD CONSTRAINT uniqueKeyName UNIQUE(columnName1, columnName2);

2. First select and then insert or update

procedure insertRow(p_key1 in number, p_key2 in number, p_value in number)
is
  cursor c1(v1 in number, v2 in number) is
     select '1' from tableName
       where columnName1 = v1
            and columnName2 = v2;
  dummy varchar2(1);
begin
  open c1(p_key1, p_key2);
  fetch c1 into dummy;
  if sql%found then
    update tableName
       set valueColumn = p_value
       where columnName1 = p_key1
            and columnName2 = p_key2;
   else
      insert into tableName (columnName1, columnName2, valueColumn)
             values (p_key1, p_key2, p_value);
   end if;
end;
/

3. The MERGE-statement

MERGE INTO tableName tn
  USING dual on (dual.dummy is not null
                 and tn.columnName1 = p_key1
                 and tn.columnName2 = p_key2)
WHEN MATCHED THEN
  UPDATE set tn.valueColumn = p_value
WHEN NOT MATCHED THEN
  insert (columnaName1, columnName2, valueColumn)
     values (p_key1, p_key2, p_value);

4. update and then insert

procedure insertRow(p_key1 in number, p_key2 in number, p_value in number)

is

begin

  update tableName
     set valueColumn = p_value
    where columnName1 = p_key1
        and columnName2 = p_key2;
  if sql%rowcount = 0 then
    insert into tableName (columnName1, columnName2, valueColumn)
        values (p_key1, p_key2, p_value);
  end if;
end;
/

or

update tableName

     set valueColumn = p_value

    where columnName1 = p_key1

        and columnName2 = p_key2;

insert into tableName (columnName1, columnName2, valueColumn)
      select p_key1, p_key2, p_value
         from dual
        where not exists (select '1'
                          from tableName
                         where columnName1 = p_key1
                           and columnName2 = p_key2);

5. first delete and then insert

procedure insertRow(p_key1 in number, p_key2 in number, p_value in number)
is
begin
  delete from tableName
     where columnName1 = p_key1
         and columnName2 = p_key2;

   insert into tableName (columnName1, columnName2, valueColumn)
        values (p_key1, p_key2, p_value);

end;
/