Hi trabelsi,
Thank you for reaching out to AdWords API and Google Ads API Forum.
We do not provide database diagrams or schemas as what you have in mind. However, we allow developers to query reports via either AdWords API or Google Ads API (Beta).
In AdWords API, you should read the Reports guide and understand the different types of reports.
Reporting Basics: https://developers.google.com/adwords/api/docs/guides/reporting
Types of reports: https://developers.google.com/adwords/api/docs/guides/reporting#choosing_the_right_report
In Google Ads API, you should use the Google Ads Query Language (GAQL) to query different resources:
GAQL: https://developers.google.com/google-ads/api/docs/query/overview
Resources: https://developers.google.com/google-ads/api/fields/latest/overview
Hope that this helps your issue, thank you.