Update Statement in ORacle based on data in another 2 tables -

i need update 1 master table based on join 2 tables. anyoe please provide me best approach here need update millions of records using update.

maybe example useful:

create table a(    id int,    str varchar2(10)); create table b(    id int,    str varchar2(10));  insert values(1,'a1'); insert values(2,'a2'); insert values(3,'a3'); insert b values(1,'b'); insert b values(3,'c'); insert b values(4,'d');  /*here query!!*/ merge using ( select id, str           b ) b on ( a.id = b.id ) when matched  update set  a.str = b.str; 

here other query, maybe have problems null values (try here):

update set str = (   select str   b   a.id = b.id); 

you can try here