SQL & PL/SQL :: Updating Current Column With Previous Columns?

I need to update the current column with sum of the previous column values. Following are the creation scripts

DROP TABLE TEST_LOG;
CREATE TABLE TEST_LOG
(

[Code]....

Above query is working fine to retrieve the previous column values.But when we are updating the SUM_PRE_COLS column with those values it's not working fine.

I tried by using the following query

UPDATE TEST_LOG T SET SUM_PRE_COLS =
( SELECT LAG(T2.KEY0, 1, 0) OVER(ORDER BY T2.KEY0) + LAG(T2.KEY1, 1, 0) OVER(ORDER BY T2.KEY0)
FROM TEST_LOG T2 WHERE T2.ROWID= T.ROWID);