SQL查询：两列中的当前和上个月值

Question

需要SQL查询将上个月的值与月度值相对应。

约束：不要使用任何LAG，OVER，PARTITION，Row_number（）等。仅使用LEFT，RIGHT或INNER连接并实现它。

我有一张桌子如下

LoanID1 Jan   2000  
LoanID1 Feb   3000  
LoanID1 March 2500  
LoanID1 April 1000  
LoanID1 May   500   
LoanID2 Jan   750   
LoanID2 FEB   3500  
LoanID2 March 2700  
LoanID2 April 1500  
LoanID2 May   4000  

基本上，这些是每个LoanID的每月值，我需要的是我需要另一个列具有上个月的值，如下表所示：

LoanID1 Jan   2000  
LoanID1 Feb   3000   2000
LoanID1 March 2500   3000
LoanID1 April 1000   2500
LoanID1 May   500    1000
LoanID2 Jan   750    
LoanID2 FEB   3500   750
LoanID2 March 2700   3500
LoanID2 April 1500   2700
LoanID2 May   4000   1500

我尝试加入同一张桌子，但无法实现。请指导我并告诉我一些如何做到这一点。

更新以提供实际细节：

插图图像：实际数据如何可用以及我在第二张图像中如何需要它

[这是具有记录的实际数据库

这是我需要的另一列（上个月的价值观）

---

感谢您的指导，并提前致谢。

Answer 1

你可以使用LAG函数来完成它

    declare @myt table (id nvarchar(50),monthnames nvarchar(50),amount int)

insert into @myt

values

('LoanID1','Jan', 2000       ),
('LoanID1','Feb', 3000       ),
('LoanID1','March',   2500   ), 
('LoanID1','April',   1000   ), 
('LoanID1','May', 500        ),
('LoanID2','Jan', 750        ),
('LoanID2','Feb', 3500       ),
('LoanID2','March',   2700   ), 
('LoanID2','April',   1500   ), 
('LoanID2','May', 4000       )

select *,LAG(Amount,1,0) Over(partition by ID order by id,MonthNumber) as PreviousMonth  from (
select *,Month(cast(monthnames+'1 2018' as date)) as MonthNumber from @myt
)x 

Answer 2

根据您使用的RDBMS，您需要添加按日期排序的“行号”列，您可以根据该列执行连接。它看起来像这样

select
t1.month,
t1.value as curr_mon_val,
t2.value as prev_mon_val
from 
(select t1.month, t1.value, row_number from t1) as t1
join (select t1.value, row_number from t1 where row_number > 1) as t2
on t1.row_number = t2.row_number-1

这是您需要编写的内容的近似值，具体取决于您的RDBMS