No, it is safest to delete the Scans from the WebInspect GUI (or with its CLI or relevant API endpoints).  This will ensure it removes all of the related data in all the Tables it was written to.  If you manually strip down Tables, you will likely orphan or break linked Tables elsewhere.  You cannot delete part of a scan in WebInspect.

Could you tell me the CLI and API to delete the scan? Thanks a lot!

the API which Hans is referring is the WebInspect API which you can activate it by following the documenation of webinspect relatated to the version you have https://www.microfocus.com/documentation/fortify-webinspect/ the steps are in the webinspect user guide at "Fortify WebInspect REST API".

Historically the SessionParameter table will be the largest because it stores ALL session data recorded during a scan. This table is used for tracking the parameters of a session.

Here is a query that will help you determine which scans have the largest number of sessions. These scans have the largest number of sessions (not necessarily largest regarding volume) to target for deletion or export is:

SELECT ScanID,
       COUNT(SessionID)AS "# of Sessions"
FROM dbo.SessionParameter
GROUP BY ScanID
ORDER BY "# of Sessions" DESC

Here is an example result from my lab.

As Hans mentioned, the recommendation is to delete any scans not needed. If you need to keep all scan for archival or auditing purposes, you can either:

• Export the scan(s) and store them in cold storage
• Create multiple databases based on a set period (month, quarter, year) and only have the latest active on the scanner(s) with the others stored in cold storage.

Have you looked at shrinking the database. The way MSSQL works is it will hold/reserve the “available” space even after deleting data. There are some pros and cons to shrinking a database. Please review https://am2.co/2016/04/shrink-database-4-easy-steps/#:~:text=FROM%20sys.database_files%20dbf%20LEFT%20JOIN%20sys.filegroups%20fg%20ON,decide%20to%20move%20on%20and%20shrink%20the%20files.

• This site lists how to shrink the database
• It also contains a query that will give you the amount of space that “may” be recovered (FreeSpaceMB) after executing the shrink command.
• Here is a link to a very informative blog post by Brent Ozar (MSSQL Expert) on https://www.brentozar.com/archive/2009/08/stop-shrinking-your-database-files-seriously-now/ regarding shrinking the database