Is it necessary to compress tables?

[Jim McAtee]

Does MySQL automatically handle deleted row cleanup, or is it necessary to
periodically do this manually for tables with a lot of deletions?  If it's
manual, what are the SQL commands to do this?

Also, what happens when changes are made to a field definition?  From, say, int
to smallint, or tinyint to int, or varchar(255) to varchar(10)?  Do temporary
tables get created and the data copied over, or does unused space end up in the
data files, to be collected later?

Thanks.