How to select the rows affected by an update
This sounds like a job for SELECT FOR UPDATE. Please see MySQL Docs on this
I have discussed this over the years

Aug 08, 2011 : Are InnoDB Deadlocks exclusive to INSERT/UPDATE/DELETE?
Jan 02, 2012 : LOCK IN SHARE MODE
Mar 18, 2012 : select for update gives error on indexed column
May 09, 2012 : Transaction Lock Timeouts When Updating a Row
May 13, 2012 : Cannot update certain rows in innodb tables
Aug 10, 2012 ; Similar function NOWAIT in MySQL
Feb 12, 2014 : row locking within ACID transaction innodb