MySQL - If slave is offline for a few hours and comes back online, does it automatically catch data from master?
In MySQL replication, a slave "syncs" its data from the master's transaction log, not directly from the database. The slave stores the last position in that transaction log, so as long as you have not touched that position number on the slave, then as soon as it becomes active again, it will resume executing transactions from that position it left off at. All should be well for you.
Here you go, for your reference:
yes - however if the master bin log has changed the slave will not be able catch it. the slave keeps track of the bin log file where the master is and the position.
you can check on master
on the slave
see if they use the same log file and note the position of the master versus slave. if there are some duplicates into the slave you can always skip records.