Restartable scripts

What is a restartable script and how do i make a script restartable?

My definition would be: after a (restartable) script has exited with an error before finishing it's work, an user/operator should be required to do only 2 things: clean up the condition for the error and start the script again. For this, the script should "clean up" if it exits, eg. leave a database in a consistent state. Also, it should not break again on things it's done before (eg. don't process data the previous run already mangled)
How to do that: depends on your application and your script. Basically, just make sure that you leave the environment in a state that you would expect when you start it again.

Thanks pludi
I am using sql loader in unix and inserting data to the database. and i have no idea as to how to make the script restart able.
an example would help a lot