You can specify sub-statuses in the WHERE clause like so:

WHERE tickets.ticket_status.id = 4

The sub-status ID can be retrieved using the API or by going to Admin > Tickets > Statuses and clicking on the Sub-status. The ID will appear in the URL bar.

Below is an example:

SELECT tickets.id, tickets.subject, tickets.ticket_status.title

FROM tickets

WHERE tickets.ticket_status.id = 3

We have used tickets.ticket_status.title in the SELECT clause to display the correct sub-status name