with m as ( select * from [meter] )
   , d as ( select id, foreign_id = isnull(foreign_id, id), fdate = min([datetime]), ldate = max([datetime]) from m where '20170214'<= [datetime] and [datetime] < '20170215' group by id, foreign_id )
select  d0.id, real_id = d.id
		,convert(varchar(20),a.datetime,120) as first_date
		,round(a.pow,1) as first_value
		,convert(varchar(20),b.datetime,120) as last_date
		,round(b.pow,1) as last_value
		,round(b.pow - a.pow,1) as sum
    from d as d0 
         inner join d on d.id = d0.foreign_id
         left outer join m as a on a.id = d.id and a.datetime = d.fdate
         left outer join m as b on b.id = d.id and b.datetime = d.ldate

with m as ( select * from [meter] )
   , d as ( select id, foreign_id = isnull(foreign_id, id), fdate = min([datetime]), ldate = max([datetime]) from m where '20170214'<= [datetime] and [datetime] < '20170215' group by id, foreign_id )
select  d0.id, real_id = d.id
		,convert(varchar(20),a.datetime,120) as first_date

		,round(a.pow,1) as first_value
		,round(a0.pow,1) as first_value0

		,convert(varchar(20),b.datetime,120) as last_date

		,round(b.pow,1) as last_value
		,round(b0.pow,1) as last_value0

		,round(b.pow - a.pow,1) as sum
    from d as d0 
         inner join d on d.id = d0.foreign_id
         left outer join m as a on a.id = d.id and a.datetime = d.fdate
         left outer join m as b on b.id = d.id and b.datetime = d.ldate

         left outer join m as a0 on a0.id = d0.id and a0.datetime = d0.fdate
         left outer join m as b0 on b0.id = d0.id and b0.datetime = d0.ldate

declare @dts table(dt date primary key);
insert @dts values('20170214'),('20170226'),('20170228');
...
where exists( select * from @dts as x where x.dt = t.[datetime])

where exists( select * from  (values('20170214'),('20170226'),('20170228') ) as x(dt) where x.dt = t.[datetime])

declare @dts table(dt date primary key);
insert @dts values('20170214'),('20170226'),('20170228');

with m as ( select * from [meter] )
   , d as ( select id, foreign_id = isnull(foreign_id, id), fdate = min([datetime]), ldate = max([datetime]) from m 
where exists( select * from @dts as x where x.dt = t.[datetime]) group by id, foreign_id 
)

declare @dts table(dt date primary key);
insert @dts values('20170214'),('20170226'),('20170228');

with m as ( select * from [meter] )
   , d as ( select id, foreign_id = isnull(foreign_id, id), fdate = min([datetime]), ldate = max([datetime]) from m 
where exists( select * from @dts as x where m.[datetime] between x.dt and dateadd(day, 1, x.dt ) ) group by id, foreign_id 
)0

declare @dts table(dt date primary key);
insert @dts values('20170214'),('20170226'),('20170228');

with m as ( select * from [meter] as m where exists( select * from @dts as x where m.[datetime] between x.dt and dateadd(day, 1, x.dt ) ) )
  select * from m;
-- должон быть результат

with m as ( select * from [meter] as m where exists( select * from @dts as x where m.[datetime] between x.dt and dateadd(day, 1, x.dt ) ))
   , d as ( select id, foreign_id = isnull(foreign_id, id), fdate = min([datetime]), ldate = max([datetime]) from m group by id, foreign_id  )
  select * from d;
-- тогда и тут будет результат

-- запросы, по первости, следует писать "понемногу" мелкими прыжками в сторону результата.