获得第一行和最后一行？

Question

我有另一个查询来选择返回最小值，最大值，开始和结束价格。 它适用于特定月份，适用于metal_id = 1，但当更改为2时，它不会返回任何数据。

请看： 下面的查询可以选择给定月份中每天的最小，最大，开始和最后价格。

我想选择相同的但是整个月，就像显示给定月份的整体表现而不是每天。

小提琴： http://sqlfiddle.com/#!9/bee86/1

我只需要最后2个价格，所选月份的第一个和最后一个价格......

select 
       highp.metal_price_datetime_IST AS high_price_metal_price_datetime_IST
     , highp.metal_price as highest_price
     , lowp.report_term
     , lowp.metal_price as lowest_price
     , lowp.metal_price_datetime_IST AS low_price_metal_price_datetime_IST
from (select @report_term:=concat(monthname(metal_price_datetime_IST), ' ', year(metal_price_datetime_IST)) as report_term
           , metal_price_datetime_IST
           , metal_price
           , metal_id
           , case when @report_term=@old_report_term then @rn1:=@rn1+1  else @rn1:=1 end as rn
           , @old_report_term:=@report_term
      from metal_prices
      cross join (select @rn1:=0, @old_report_term:='') inituservar1
      where datediff(now(), metal_price_datetime_IST) between 0 and 180
     and metal_id = 1
      order by metal_id, report_term, metal_price asc) lowp
inner join (select @report_term2:=concat(monthname(metal_price_datetime_IST), ' ', year(metal_price_datetime_IST)) as report_term
           , metal_price_datetime_IST
           , metal_price
           , metal_id
           , case when @report_term2=@old_report_term2 then @rn2:=@rn2+1  else @rn2:=1 end as rn
           , @old_report_term2:=@report_term2
      from metal_prices
      cross join (select @rn2:=0, @old_report_term2:='') inituservar1
      where  datediff(now(), metal_price_datetime_IST) between 0 and 180
            and metal_id = 1
      order by metal_id, report_term, metal_price desc) highp
 on lowp.rn=highp.rn
 and lowp.metal_id = highp.metal_id
 and lowp.report_term = highp.report_term
 and lowp.rn = 1

 order by lowp.metal_price_datetime_IST DESC