Question

I created a table “Test”

create table test
(
  id int identity(1,1) not null,
  correlation int,
  data varchar(max)
)

Below are data of the table

insert into test(correlation,data) values(1,'x0')
insert into test(correlation,data) values(1,'x1')
insert into test(correlation,data) values(2,'z1')
insert into test(correlation,data) values(2,'z2')
insert into test(correlation,data) values(3,'a')
insert into test(correlation,data) values(4,'b')
insert into test(correlation,data) values(5,'c')

I need to display data on the web page and to connect table to itself on
correlation and to do paging

For example if I have two records with the same correlation (1) I need to
display two rows as one row with the data as Current Data and Previous Data.
In example below current data will be x1 and previous will be x0.

Before

 Correlation  Data
    1         x0    
    1         x1    

After

Correlation     Previous Data   Current Data
1                  xo             x1

If correlation has only one row , than previous correlation in result will be null.

Currenly I did paging in Linq and it’s working but I am afraid that in future it will cost
performance problem.

Can sameone can help me with SQL .

Is there other good solution for this problem.

Answer 1

;with C as
(
  select correlation,
         data,
         row_number() over(partition by correlation order by id desc) as rn
  from @test
  where SomeColumn > 10 -- Where clause goes here (if possible)
)  
select C1.correlation,
       C2.data as [Previous Data],
       C1.data as [Current Data]
from C as C1
  left outer join C as C2 
    on C1.correlation = C2.correlation and
       C2.rn = 2
where C1.rn = 1

Result:

correlation Previous Data Current Data
----------- ------------- ------------
1           x0            x1
2           z1            z2
3           NULL          a
4           NULL          b
5           NULL          c