First hit from most search engines is https://developer.atlassian.com/server/jira/platform/database-schema/ which gives you the database scheme for 6 and 7. 8 is not documented yet, and probably never will be, as Server is going away. The scheme given for 7 is almost entirely valid for 8 though, the changes from 7 to 8 are trivial.
Of course, this also begs the question of why you want it - using SQL to read a Jira database is the worst possible way of accessing the data, it's simply not designed to be reported off. One simple example - "show me an issue" - it's a single REST call that will give you all the issue data in one JSON response. Or, for an issue with no custom fields, history or comments, only 18 joins in your SQL (obviously a lot more if you want the custom fields, history or comments, and you'll have to write more code if you want to see any of the calculated stuff)
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