Could somebody please assist me

if i write update statement without any where clause, it is full table scan,

here the in my exist stable they have about 7 million rows, and also in new table (where update/insert take place) will have these many rows (after first insert/update),

rather than issueing stright update command, can we change the logic (functioanlly same) to include where clause etc. to restrict full table scan etc.,

or if you suggest it is good then i will go ahead

please advice me, also with merge statement

thank you

dhani