How to make a comparison of the amount for the data of this year and the past in two different columns? Here is my code. Returns null values:

SELECT a.did_dat(дата),a.pre_kod(товар), a.pre_ska(количество),a.kli_kod(клиент), b.did_dat-1(дата прошедшего года),b.pre_ska (количество прошлого года) FROM ( SELECT kli_kod,pre_kod,did_dat,pre_ska FROM SVF_View_10 ) AS a LEFT OUTER JOIN ( SELECT pre_kod,kli_kod,did_dat,pre_ska FROM dbo.SVF_View_10_sum WHERE did_dat = DATEADD(YEAR, -1, did_dat) ) AS b ON a.kli_kod = b.kli_kod AND a.pre_kod = b.pre_kod AND a.did_dat = b.did_dat;

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