SQL Server 2019

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   create table #Schedules (
        ScheduleId int,
        ScheduleName varchar(20)
        );

    create table #ScheduleThings (
        ScheduleThingId int,
        ScheduleId int,
        Thing decimal(18,2));

    create table #ScheduleOtherThings (
        ScheduleOtherThingId int,
        ScheduleId int,
        OtherThing varchar(50));

    insert into #Schedules (
        ScheduleId,
        ScheduleName )
    values 
        (1, 'A'),
        (2, 'B');

    insert into #ScheduleThings (
        ScheduleThingId,
        ScheduleId,
        Thing )
    values
       (1, 1, 10.22),
       (2, 1, 11.02),
       (3, 1, 11.89),
       (1, 2, 19.23),
       (2, 2, 20.04),
       (3, 2, 20.76),
       (4, 2, 21.37);

    insert into #ScheduleOtherThings (
        ScheduleOtherThingId,
        ScheduleId,
        OtherThing )
    values
        (1, 1, 'Always'),
        (2, 1, 'Sometimes'),
        (3, 2, 'Seldom'),
        (4, 2, 'Always'),
        (5, 2, 'Never');
14 rows affected

    select
      fr.result
    from
    (
      select
        s.ScheduleId             as [schedules.schedule_id],
        s.ScheduleName           as [schedules.schedule_name],
        (
          SELECT ScheduleThingId, Thing
            FROM #ScheduleThings
           WHERE ScheduleId = s.ScheduleId
             AND ScheduleThingId < 3
             FOR JSON PATH
        )
          AS [schedules.schedule_things],
        (
          SELECT ScheduleOtherThingId, OtherThing
            FROM #ScheduleOtherThings
           WHERE ScheduleId = s.ScheduleId
             FOR JSON PATH
        )
          AS [schedules.schedule_other_things]
      from
        #Schedules s
      where
        s.ScheduleId = 1
      for json path, root('schedules') 
   )
     fr(result) ;


result


{"schedules":[{"schedules":{"schedule_id":1,"schedule_name":"A","schedule_things":[{"ScheduleThingId":1,"Thing":10.22},{"ScheduleThingId":2,"Thing":11.02}],"schedule_other_things":[{"ScheduleOtherThingId":1,"OtherThing":"Always"},{"ScheduleOtherThingId":2,"OtherThing":"Sometimes"}]}}]}


    select fr.result from (
    select
        s.ScheduleId                                 as [schedule_id],
        s.ScheduleName                               as [schedule_name],
        schedule_things.ScheduleThingId              as [schedule_thing_id],
        schedule_things.Thing                        as [thing],
        schedule_other_things.ScheduleOtherThingId   as [schedule_other_thing_id],
        schedule_other_things.OtherThing             as [other_thing]
      from #Schedules s
      join #ScheduleThings schedule_things
        on schedule_things.ScheduleId = s.ScheduleId
      join #ScheduleOtherThings schedule_other_things
        on schedule_other_things.ScheduleId = s.ScheduleId
     where s.ScheduleId = 1
       and schedule_things.ScheduleThingId < 3
      for json auto, root('schedules') ) fr(result) ;


result


{"schedules":[{"schedule_id":1,"schedule_name":"A","schedule_things":[{"schedule_thing_id":1,"thing":10.22,"schedule_other_things":[{"schedule_other_thing_id":1,"other_thing":"Always"},{"schedule_other_thing_id":2,"other_thing":"Sometimes"}]},{"schedule_thing_id":2,"thing":11.02,"schedule_other_things":[{"schedule_other_thing_id":1,"other_thing":"Always"},{"schedule_other_thing_id":2,"other_thing":"Sometimes"}]}]}]}