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:
Badges are a great way to show off community activity, whether you’re a newbie or a Champion.Learn more
Hi Atlassian Community! My name is Shana, and I’m on the Jira Software team. One of the many reasons this Community exists is to connect you to others on similar product journeys or with comparabl...
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
You're one step closer to meeting fellow Atlassian users at your local meet up. Learn more about AUGs