Question

I’m wondering if anyone’s come across a neat solution to this problem. I’m trying to select data from a couple of tables, having the records match up row by row. I’m basically after a full outer join, but there’s one crucial difference. If I have four rows with a particular value in the column I’m joining on in one table, and three rows with this value in another, I only want the first three results to be joined, and the fourth to act as if there had been no match.

The reason for this is to create a reconciliation report which ensures transactions are not counted multiple times when comparing results. I can get around this issue by using a bit of grouping and some aggregate functions, but this hides some of the detail which I’d like to keep.

Below is an example to show the sort of thing I’m after, with the invalid/pseudo code in the comments illustrating how I’m thinking of this as working:

declare @t1 table (id bigint identity(1,1) primary key clustered, foreignKeyId bigint, otherData nvarchar(10))
declare @t2 table (id bigint identity(1,1) primary key clustered, foreignKeyId bigint, moreData nvarchar(10))

insert @t1 select 1, '1.1.1'
 union all select 1, '1.1.2'
 union all select 1, '1.1.3'
 union all select 3, '1.3.1'
 union all select 3, '1.3.2'
 union all select 3, '1.3.3'
 union all select 4, '1.4.3'

insert @t2 select 1, '2.1.1'
 union all select 1, '2.1.2'
 union all select 1, '2.1.3'
 union all select 2, '2.2.1'
 union all select 3, '2.3.1'
 union all select 3, '2.3.2'
 union all select 5, '2.5.1'
 union all select 5, '2.5.2'

--demo of the functionality i'm hoping to acheive 
--
 /*
 select t1.id           id1
 , t2.id                id2
 , t1.foreignKeyId  fk1
 , t2.foreignKeyId  fk2
 , t1.otherData     otherData
 , t2.moreData      moreData
 from @t1 t1
 full funky join @t2 t2
 on t1.foreignKeyId = t2.foreignKeyId
 order by t1.id, t2.id --we'd need an order by to ensure the match could be applied in a predictable manner
 */
--
declare @funkyjoin table (id1 bigint, id2 bigint, fk1 bigint, fk2 bigint, otherData nvarchar(10), moreData nvarchar(10))
declare @id1 bigint, @id2 bigint
insert @funkyjoin (id1, fk1, otherData) 
select id, foreignKeyId, otherData from @t1

while exists(select 1 from @t2)
begin
    select top 1 @id2 = id from @t2 order by id 

    set @id1 = null

    select top 1 @id1 = id1 
    from @funkyjoin
    where fk2 is null
    and fk1 in (select foreignKeyId from @t2 where id = @id2)

    if @id1 is null
    begin
        insert @funkyjoin (id2, fk2, moreData)
        select id, foreignKeyId, moreData
        from @t2
        where id = @id2
    end
    else
    begin
        update @funkyjoin
        set id2 = @id2
        , fk2 = fk1 --since we're joining on this we can just match it
        , moreData = (select moreData from @t2 where id = @id2)
        where id1 = @id1 
    end

    delete from @t2 where id = @id2 --since this is only an example let's not worry about keeping our source data
end

select * 
from @funkyjoin
order by coalesce(id1, id2)

I’ve written a similar solution for when this scenario occurs on spreadsheets previously: http://officemacros.codeplex.com/#WorksheetMergeMacro

Answer 1

If I understand correctly, this may be what you’re after:

select *
from (
  select *,
    row_number() over (partition by foreignKeyId order by id) as n
  from @t1
) t1
full outer join (
  select *,
    row_number() over (partition by foreignKeyId order by id) as n
  from @t2
) t2 on t1.foreignKeyId = t2.foreignKeyId and t1.n = t2.n