JQL for sub-tasks works the same in Next-Gen as Classic.
You can search for all sub-tasks in one story using this query:
parent = STORY-1
^ This will return all the sub-tasks which exist under a story.
Or you can use:
linkedIssue = STORY-1
^ This will return all sub-tasks and the story itself. You can change STORY-1 to EPIC-1 to return all stories / sub-tasks in an Epic.
If you're searching by just one story, there should be no need for sprint or project.
If however you have a more complex query - such as providing all the sub-tasks below all stories in a specific sprint, you'd need to either search for each story individually...
parent = STORY-1 or parent = STORY-2 or parent = STORY-3
...or use an app which expands the power of JQL, to allow you to search for all sub-tasks below issues which match an additional sub-query.
Let us know if any of these work for you - or if you need a more advanced query, let me know the specifics and I'll try to suggest an alternative :)
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