You can check documentation for REST API here: https://docs.atlassian.com/jira/REST/latest/
Though you don't have access to the database, you can create a ticket at Atlassian Support and we'll be glad to help you out with any inquiries or problem you may have.
Jaime Kirch da Silveira
Atlassian Cloud Support
What i actually need is that at Create Issue, Priority + Urgency will result in a new Severity issue with a metric how severe an issue is. I have found a case how to do it (https://confluence.atlassian.com/display/JIRA/Setting+Priority+field+value+based+on+customfield+value)
If you want to know why we don't allow it ask yourself this question: would you expect GMail to give you database access? It is just a counter intuitive thing to allow for a service that is supposed to be managed for you. I would also recommend using the REST API's; if they can't do what you need them to then let us know and we'll look into it. I'd be very surprised to hear that you could not do somehing via the REST API's though. They are pretty comprehensive: https://docs.atlassian.com/jira/REST/latest
Well there are some parts of the JIRA documentation that point you towards changing values in the database, for example:
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