Check value exist in previous month in SQL

Question

I have two table XYZ and ABC

create table #abc(id int, value int, dates date)
create table #xyz(id int, value int, dates date)

insert into #xyz values(1,10,'2017/02/01')
Insert into #xyz values(2,10,'2017/05/01')

Insert into #abc values(1,10,'2017/01/01')
Insert into #abc values(1,20,'2017/02/01')
Insert into #abc values(2,4,'2017/01/01')
Insert into #abc values(2,5,'2017/04/01')

XYZ
ID  Value   Dates
1   10  2017/02/01
2   10  2017/05/01

ABC         
ID  Values  Dates   ExpectedResult
1   10  2017/01/01  20
1   20  2017/02/01  0
2   4   2017/01/01  0
2   5   2017/04/01  15

i want to check if table ABC having (Month - 1 ) value comparing with table XYZ. Uniqueness of table is identified by ID and Date . For eg-> Table XYZ id-1 is having date 2017/02/01 ie February data and my ABC table ID-1 is having 2017/01/01 ie January data then XYZ value + ABC value ie 20 is the expected result.

Could you please tell me how do we compare month in this scenario.

Answer 1

You need to use the DATEADD function provided by MS SQL to compare dates, for example

WHERE XYZ.Date = DATEADD(MONTH, -1, ABC.Date)

Have a look at this link on w3schools for a detailed description