It's just plain ol' MYSQL with a few Joomla/CB additives (mainly #__ as prefix and substitutions). So if you've an error then it's likely with your host and their MYSQL server. The CB Query Field plugin simply provides a new feidltype where you can provide a simply select query. Example as follows.
Code:
SELECT `username` FROM `#__users` WHERE `id` = [user_id]
Would be able to better suggest what's happening with the error you're seeing provided.
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