我正在尝试在MSSQL中使用LAG（）函数，并且出现了奇怪的行为。 该表如下所示：
ID TotalReadings Month Device
0 1 4 January M
1 1 4 January D
2 1 4 January T
2 1 4 January L
2 1 2 February M
2 1 2 February D
2 1 2 February L
0 1 2 February T
1 1 6 March M
2 1 6 March D
2 1 6 March L
2 1 6 March T
2 1 6 April M
2 1 6 April D
2 1 6 April T
2 1 6 April L
我所做的是：
Select *,
CASE
WHEN
ISNULL(LAG(TotalReadings) OVER (PARTITION BY ID ORDER BY Month ), 0) < TotalReadings THEN 'Increase'
WHEN
ISNULL(LAG(TotalReadings) OVER (PARTITION BY ID ORDER BY Month), 0) = TotalReadings THEN 'Neutral'
WHEN
ISNULL(LAG(TotalReadings) OVER (PARTITION BY ID ORDER BY Month), 0) > TotalReadings THEN 'Decrease'
END As Trend
from table
并得到：
ID TotalReadings Month Device Trend
0 1 4 January M Increase
1 1 4 January D Neutral
2 1 4 January T Neutral
2 1 4 January L Neutral
2 1 2 February M Decrease
2 1 2 February D Neutral
2 1 2 February L Neutral
0 1 2 February T Neutral
1 1 6 March M Increase
2 1 6 March D Neutral
2 1 6 March L Neutral
2 1 6 March T Neutral
2 1 6 April M Neutral
2 1 6 April D Neutral
2 1 6 April T Neutral
2 1 6 April L Neutral
但是我真正想要的是首先按月分组，并将趋势命名为“开始”，因为没有可与之比较的先前值，而剩余部分应考虑类似的TOtalReadings，每月总计相同，因此趋势不应该只针对新月行开始的第一行，而是所有趋势：
ID TotalReadings Month Device Trend
0 1 4 January M Start
1 1 4 January D Start
2 1 4 January T Start
2 1 4 January L Start
2 1 2 February M Decrease
2 1 2 February D Decrease
2 1 2 February L Decrease
0 1 2 February T Decrease
1 1 6 March M Increase
2 1 6 March D Increase
2 1 6 March L Increase
2 1 6 March T Increase
2 1 6 April M Neutral
2 1 6 April D Neutral
2 1 6 April T Neutral
2 1 6 April L Neutral
有任何线索吗？
答案 0 :(得分：0)
您在这里：
create table #t
(id int, totalreadings int, month int, device char(1))
insert into #t
values
(1,4,1,'M'),
(1,4,1,'D'),
(1,4,1,'T'),
(1,4,1,'L'),
(1,2,2,'M'),
(1,2,2,'D'),
(1,2,2,'L'),
(1,2,2,'T'),
(1,6,3,'M'),
(1,6,3,'D'),
(1,6,3,'L'),
(1,6,3,'T'),
(1,6,4,'M'),
(1,6,4,'D'),
(1,6,4,'L'),
(1,6,4,'T')
Select *,
CASE
WHEN
LAG(TotalReadings) OVER (PARTITION BY ID,device ORDER BY Month ) < TotalReadings THEN 'Increase'
WHEN
LAG(TotalReadings) OVER (PARTITION BY ID,device ORDER BY Month) = TotalReadings THEN 'Neutral'
WHEN
LAG(TotalReadings) OVER (PARTITION BY ID,device ORDER BY Month) > TotalReadings THEN 'Decrease'
ELSE 'Start'
END As Trend
from #t
order by month
id totalreadings month device Trend
1 4 1 D Start
1 4 1 L Start
1 4 1 M Start
1 4 1 T Start
1 2 2 T Decrease
1 2 2 M Decrease
1 2 2 L Decrease
1 2 2 D Decrease
1 6 3 D Increase
1 6 3 L Increase
1 6 3 M Increase
1 6 3 T Increase
1 6 4 T Neutral
1 6 4 M Neutral
1 6 4 L Neutral
1 6 4 D Neutral
答案 1 :(得分：0)
如果添加identity列，则可以使用此代码
create table #order ( i int identity(1,1), ID int, TotalReadings int, Month varchar(20), Device varchar(1))
insert #order values
( 1 , 4 ,'January' ,'M' )
,( 1 , 4 ,'January' ,'D' )
,( 1 , 4 ,'January' ,'T' )
,( 1 , 4 ,'January' ,'L' )
,( 1 , 2 ,'February' ,'M' )
,( 1 , 2 ,'February' ,'D' )
,( 1 , 2 ,'February' ,'L' )
,( 1 , 2 ,'February' ,'T' )
,( 1 , 6 ,'March' ,'M' )
,( 1 , 6 ,'March' ,'D' )
,( 1 , 6 ,'March' ,'L' )
,( 1 , 6 ,'March' ,'T' )
,( 1 , 6 ,'April' ,'M' )
,( 1 , 6 ,'April' ,'D' )
,( 1 , 6 ,'April' ,'T' )
,( 1 , 6 ,'April' ,'L' )
Select *
,CASE
WHEN
lag(TotalReadings, 4) OVER (PARTITION BY ID ORDER BY i,id, Month) < TotalReadings THEN 'Increase'
WHEN
lag(TotalReadings, 4) OVER (PARTITION BY ID ORDER BY i,id, Month) = TotalReadings THEN 'Neutral'
WHEN
lag(TotalReadings, 4) OVER (PARTITION BY ID ORDER BY i,id, Month) > TotalReadings THEN 'Decrease'
WHEN
lag(TotalReadings, 4) OVER (PARTITION BY ID ORDER BY i,id, Month) is null THEN 'start'
END As Trend
from #order
order by i
编辑1：无需身份列
Select *
,CASE
WHEN
lag(TotalReadings, 4) OVER (PARTITION BY ID ORDER BY id, MONTH(MONTH + ' 1 2014') ) < TotalReadings THEN 'Increase'
WHEN
lag(TotalReadings, 4) OVER (PARTITION BY ID ORDER BY id, MONTH(Month + ' 1 2014') ) = TotalReadings THEN 'Neutral'
WHEN
lag(TotalReadings, 4) OVER (PARTITION BY ID ORDER BY id, MONTH(Month + ' 1 2014') ) > TotalReadings THEN 'Decrease'
WHEN
lag(TotalReadings, 4) OVER (PARTITION BY ID ORDER BY id, MONTH(Month + ' 1 2014') ) is null THEN 'start'
END As Trend
from #order
order by MONTH(Month + ' 1 2014')