SQL Select one row over a matching row from two tables - sql

Question

I have two tables with the same fields, but a final value that is calculated slightly differently. I need to combine the data from these two tables into one but need to prioritise one record over another when there is a match. Do you know how this might be possible?
Below is a mock up of two matching records:
ID Balance Type CCY Payment Final_Balance
28 1068376.037 F - CC GBP 78124 990252.0367
28 1068376.037 F - DD GBP 982905 85470.08293
Apologies if the format comes out poorly, I'm unsure how to format table data.
I have thousands of records in these two tables but for a handful of records I have the same information in both tables. Essentially what I'm trying to get to is where there is a match I want it to select F-CC over F-DD so I end up with unique records in my final table.
Thanks

Answer 1

I personally use ROW_NUMBER() for things like this, but there may be a better solution.
You can re-run this SQL to show how the final answer is slowly built up:
declare #t1 table (id int)
declare #t2 table (id int, txt varchar(2))
insert into #t1
select 1 union
select 2
insert into #t2
select 1, 'FC' union
select 1, 'FD' union
select 2, 'FC' union
select 2, 'FD'
select *, row_number() over (partition by id order by txt) as we_want_the_ones
from #t2
select * from (
select id, txt, row_number() over (partition by id order by txt) as we_want_the_ones
from #t2
) z
where we_want_the_ones = 1
select *
from #t1 a
join (
select * from (
select id, txt, row_number() over (partition by id order by txt) as we_want_the_ones
from #t2
) z
where we_want_the_ones = 1
) b on a.id = b.id

Answer 2

My understanding of the question is that you have two tables (A and B) which have the exact same columns. You want to UNION these tables into one dataset, but sometimes you have rows in the two tables which "match" each other. In this case you only take one of the rows based on some priority.
From your example it seems that..
Match: Occurs when the ID is the same.
Priority: Is based on the Type column, prioritized by lower alphabetical order.
Also I'm assuming SQL Server, since that's what I prefer and you didn't say.
Hopefully all that is correct.. Now, here is how I would approach it.
I would start by performing the UNION of the two tables. Taking all records and not worrying about matching yet, putting them in a temp table to use later.
SELECT ID, Balance, Type, CCY, Payment, Final_Balance
INTO #AllRecords
FROM A
UNION
SELECT ID, Balance, Type, CCY, Payment, Final_Balance
FROM B
Next, I would GROUP BY the fields which determine a match, then use MIN or MAX to get the correct value for priority columns. By my understanding of your problem that means..
SELECT ID, MIN(Type) AS Type
FROM #AllRecords
GROUP BY ID
With that query you now have the natural key for all the records you want to display in your final result. All that is left to do is look up the rest of the columns using those keys, we can do this by using that query as a subquery.
SELECT ID, Balance, Type, CCY, Payment, Final_Balance
FROM #AllRecords r
INNER JOIN (
SELECT ID, MIN(Type) AS Type
FROM #AllRecords
GROUP BY ID ) final ON r.ID = final.ID AND r.Type = final.Type
So all together the resulting query is..
SELECT ID, Balance, Type, CCY, Payment, Final_Balance
INTO #AllRecords
FROM A
UNION
SELECT ID, Balance, Type, CCY, Payment, Final_Balance
FROM B
SELECT ID, Balance, Type, CCY, Payment, Final_Balance
FROM #AllRecords r
INNER JOIN (
SELECT ID, MIN(Type) AS Type
FROM #AllRecords
GROUP BY ID ) final ON r.ID = final.ID AND r.Type = final.Type