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I very strongly recommend that you do not do this. Jira's database is not in the slightest bit suitable for reporting, and you are not going to understand how it works in order to be able to build any useful reporting for months, if not years. Or reporting off it without crippling the database performance either.
As an example, to read a single issue as the user sees it in the UI, you have to make something like 18 joins just to get the basics, many 3 layers deep. And that's before you look at history or custom fields.
Please, look in the marketplace, people have solved this problem for you - EazyBI has a thorough understanding of the data, and does all the integration work for you. If you just want something that provides (sane) data for your own reporting software, look to the "connector" apps in the marketplace (PowerBI connector for example)
Or, if you are only looking for specific reports, take a look at the other reporting apps, they might have something of use to you!
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