====== update by joining tables ======
You want to update one table with values from another (make sure you want to update and not insert).  Each row corresponds to one row in another table, so a join needs to be used.  Use 'from' with the update statement.
<code sql>
update TableToBeModified
set fieldA = t.fieldA
from TableToBeJoined t
where TableToBeModified.fieldB = t.fieldB
</code>

There are several gotchas for this:
  * TableToBeModified cannot be aliased, so use its full name
  * only one table can be updated at a time, so you can't say "set t.fieldC = true" in the query above