Reporting Services has a system database called "ReportServer" (default) which it uses to store the information related to reports, data sources, report execution etc. stored/accessed on the Report Server.
A table called "Catalog" stores the information related to various objects hosted on the Report Server like Reports, Linked Reports, Data Sources etc.
A table called "ExecutionLogStorage" stores the information related to execution/generation of reports etc.
When a report is added to Report Server, an entry will be made into "Catalog" table and when it is executed, an entry is made into "ExecutionLogStorage" table. Interestingly, when a report is deleted from the Report Server, the entry from "Catalog" table is deleted but the entry in the "ExecutionLogStorage" table is still retained.
Use the following query to verify it on your own.
WHERE ReportID NOT IN (SELECT ItemID FROM Catalog WHERE Type = 2)
Note: Table names are as per the SSRS 2008 version.
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