Files that are compressed make downloading them easier. But what do I do once I've downloaded my compressed .tar.gz file?

You have some choices when it comes to decompressing these files.
If you're on a Linux machine and you want to decompress a file in the current working directory, you can execute this command

$ tar -xvz drupal-6.x-dev.tar.gz


Finding records in one table not present in another table you could use either of these two methods.
For example, you want to see all of the registered users who have yet to place an order.

SELECT users.*
FROM users
LEFT JOIN uc_orders ON users.uid = uc_orders.uid
WHERE uc_orders.uid IS NULL

Or if joins are not supported you could use the slower method


SELECT users.*
FROM users
WHERE users.uid NOT IN (SELECT uid FROM uc_orders)