Updating a table using joins in Oracle

1. Scalar sub query
   UPDATE x SET foo = (SELECT y.foo FROM y WHERE y.bar = x.bar);
   
2. Scalar sub query with existential predicate
   UPDATE x SET foo = ( SELECT y.foo FROM y WHERE y.bar = x.bar) WHERE EXISTS ( SELECT y.foo FROM y WHERE y.bar = x.bar);
   
3. Updatable view (requires foreign key from x to y)
   UPDATE ( SELECT x.foo AS old_foo, y.foo AS new_foo FROM x, y WHERE y.bar = x.bar) SET old_foo = new_foo;
   
4. Merge Update
   MERGE INTO x USING (SELECT foo, bar FROM y ) y ON (y.bar = x.bar) WHEN matched THEN UPDATE SET x.foo = y.foo;
   Note: WHEN NOT MATCHED construct is optional