Hi Mortoncj,
I have used below screenshot for query case with not open activity, but it still bring up cases with tasks that open, and cases without any tasks can't be returned.

For example, case-qw1 has two tasks, one is open, another is completed, it has been returned with above query.

Maybe you can do two steps to achieve it indirectly.
(1)Query all cases that are assigned to specific user.

(2)Query all cases that are assigned to specific user and related task is open.

So qw3 and qw4 are cases you want.
Regards,
Leah Ju
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