解释一下这段代码 with a as ( SELECT CONVERT(nvarchar(10), getdate() ,120) AS datatime,1 as sortnum union all SELECT CONVERT(nvarchar(10), DATEADD(day, -1, getdate()) ,120) AS datatime,2 as sortnum union all SELECT CONVERT(nvarchar(10), DATEADD(day, -2, getdate()) ,120) AS datatime,3 as sortnum union all SELECT CONVERT(nvarchar(10), DATEADD(day, -3, getdate()) ,120) AS datatime,4 as sortnum union all SELECT CONVERT(nvarchar(10), DATEADD(day, -4, getdate()) ,120) AS datatime,5 as sortnum union all SELECT CONVERT(nvarchar(10), DATEADD(day, -5, getdate()) ,120) AS datatime,6 as sortnum union all SELECT CONVERT(nvarchar(10), DATEADD(day, -6, getdate()) ,120) AS datatime,7 as sortnum ), b as ( select GET_DATE,TAG_VALUE from T_TAG_DATA where TAG_ID = 1408 ) , -- 当日产量 c as ( select a.datatime,ISNULL(sum(b.TAG_VALUE),0) as 当日产量 from a left join b on a.datatime = SUBSTRING(CONVERT(varchar(100), b.GET_DATE, 120),1,10) GROUP BY a.datatime ), -- 台时产量 d as( select a.datatime,b.TAG_VALUE as 台时产量 from a left join b on a.datatime = SUBSTRING(CONVERT(varchar(100), b.GET_DATE, 120),1,10) where SUBSTRING(CONVERT(varchar,GETDATE(),120),12,2) = SUBSTRING(CONVERT(varchar(100), b.GET_DATE, 120),12,2) ), -- 累计产量 e as ( SELECT CONVERT(nvarchar(10), getdate() ,120) AS datatime,sum(TAG_VALUE) as 累计产量 from b where GET_DATE <= getdate() union all SELECT CONVERT(nvarchar(10), DATEADD(day, -1, getdate()) ,120) AS datatime,sum(TAG_VALUE) as 累计产量 from b where GET_DATE <= DATEADD(day, -1, getdate()) union all SELECT CONVERT(nvarchar(10), DATEADD(day, -2, getdate()) ,120) AS datatime,sum(TAG_VALUE) as 累计产量 from b where GET_DATE <= DATEADD(day, -2, getdate()) union all SELECT CONVERT(nvarchar(10), DATEADD(day, -3, getdate()) ,120) AS datatime,sum(TAG_VALUE) as 累计产量 from b where GET_DATE <= DATEADD(day, -3, getdate()) union all SELECT CONVERT(nvarchar(10), DATEADD(day, -4, getdate()) ,120) AS datatime,sum(TAG_VALUE) as 累计产量 from b where GET_DATE <= DATEADD(day, -4, getdate()) union all SELECT CONVERT(nvarchar(10), DATEADD(day, -5, getdate()) ,120) AS datatime,sum(TAG_VALUE) as 累计产量 from b where GET_DATE <= DATEADD(day, -5, getdate()) union all SELECT CONVERT(nvarchar(10), DATEADD(day, -6, getdate()) ,120) AS datatime,sum(TAG_VALUE) as 累计产量 from b where GET_DATE <= DATEADD(day, -6, getdate()) ) select SUBSTRING(a.datatime,6,10) as datatime,ISNULL(c.当日产量,0) as 当日产量,ISNULL(d.台时产量,0) as 台时产量,ISNULL(e.累计产量,0) as 累计产量 from a left join c on a.datatime = c.datatime left join d on a.datatime = d.datatime left join e on a.datatime = e.datatime order by a.datatime asc