Does running cron use bandwidth?
Not as such.
(technically, you could conceivably use cURL to fetch pages your own website or something similar that would end up using bandwidth, but I don’t think this is what you mean)
What are you intending to do in the cron job that would make you think you’d use bandwidth?
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Though it generally doesn’t use bandwidth, it does use CPU resources. Naturally, if you go overboard with something (such as server CPU load), they’ll let you know.
I have it run a SQL file, and I would like to run that file every 30 minutes… It is a 1 line SQL statement.
If it’s a lightweight SQL file, then there’s no worry. That’s just a blip on the database server if it’s a set of simple SQL commands. And, yes, they do monitor DB servers for heavy loads. Those stats are also available to you in the Panel under Status -> MySQL Usage.