Thread_entry in reports; show only private notes

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Hi,

a requirement is to show only the private notes of an Incident in a report. I managed to squeeze all the threads.note s into one field with group_concat:

group_concat(threads.note, ';', threads.entered, ASC)

But how can I only show the private notes?

I tried this:

if(threads.entry_type = 1, group_concat(threads.note, ';', threads.entered, ASC), '')

But the result is "second and third argument must match datatype (found blob and string)". However, there is no function to convert to blob or to string or to create an empty blob?