MySQL

Syntax

alter table Airline add unique key code (code);
alter table Airline drop key  code;

innodb_file_per_table

Each table will have its own tablespace instead of using a common tablespace. This is useful if you are low on space - you can drop at table and disk space will be reclaimed. If you are using a common tablespace then its not possible. Usually database servers are allocated space beforehand, so this is more of a developer setup. If you were to do this on production I'm unsure of the performance impacts.