Question

My table records is like below

ym  cnt
200901  57
200902  62
200903  67
...
201001  84
201002  75
201003  75
...
201101  79
201102  77
201103  80
...

I want to computer the diff between current month and per month .
the result would like below …

ym  cnt       diff
200901  57        57   
200902  62        5  (62 - 57)
200903  67        5  (67 - 62) 
...
201001  84       ...
201002  75
201003  75
...
201101  79
201102  77
201103  80
...

Can anyone told me how to wrote a sql to got the result and with a good performance ?

UPDATE:

sorry for simple words

my solution is

step1: input the currentmonth data into temp table1

step2: input the permonth data into temp table2

step3: left join 2 tables to compute the result 

Temp_Table1

SELECT  (ym - 1) as ym , COUNT( item_cnt ) as cnt
FROM _table 
GROUP BY (ym - 1 ) 
order by ym

Temp_Table2

SELECT  ym ,  COUNT( item_cnt ) as cnt
FROM _table
GROUP BY ym 
order by ym



select ym , (b.cnt - a.cnt) as diff from Temp_Table2 a 
           left join Temp_Table1 b
                on a.ym = b.ym

*If i want to compare the diff between the month in this year and last year
I can only change the ym – 1 to ym – 100*

but , actually , the group by key is not only ym

there is max 15 keys and max 100 millions records

so , I wonder a good solution can easy to manager the source

and good performance.

Answer 1

For MSSQL, this has one reference to the table, so potentially it can be faster (maybe not) than left join which has two references to the table:

-- ================
-- sample data
-- ================
declare @t table
(
    ym varchar(6),
    cnt int
)

insert into @t values ('200901', 57)
insert into @t values ('200902', 62)
insert into @t values ('200903', 67)
insert into @t values ('201001', 84)
insert into @t values ('201002', 75)
insert into @t values ('201003', 75)

-- ===========================
-- solution
-- ===========================
select
    ym2,
    diff = case when cnt1 is null then cnt2
        when cnt2 is null then cnt1
        else cnt2 - cnt1
        end
from
(
    select
        ym1 = max(case when k = 2 then ym end),
        cnt1 = max(case when k = 2 then cnt end),
        ym2 = max(case when k = 1 then ym end),
        cnt2 = max(case when k = 1 then cnt end)
    from
    (
        select 
            *, 
            rn = row_number() over(order by ym)
        from @t
    ) t1
    cross join
    (
        select k = 1 union all select k = 2
    ) t2
    group by rn + k
) t
where ym2 is not null