Join tables but allow use of records once only - sql-server

Question

CREATE TABLE #A (UpperLimit NUMERIC(4))
CREATE TABLE #B (Id NUMERIC(4), Amount NUMERIC(4))
INSERT INTO #A VALUES
(1000), (2000), (3000)
INSERT INTO #B VALUES
(1, 3100),
(2, 1900),
(3, 1800),
(4, 1700),
(5, 900),
(6, 800)
Given these 2 tables, I want to join Table A to B ON B.Amount < A.UpperLimit but each record from Table B can only be used once, so the desired output would be:
I could easily do this by plopping Table B's records into a temp table, cursor over table A taking top record < UpperLimit and Deleting that record from the temp table or some other programmatic solution, but I'd like to avoid that and I'm pretty sure this could be done with a "normal" (recursive CTE? Partition?) query.

Answer 1

You could achieve your desired output using below recursive CTE
WITH
DATA AS
(
SELECT * FROM #A A1 INNER JOIN #B B1 ON A1.UpperLimit >= B1.Amount
),
MA AS
(
SELECT MIN(UpperLimit) AS MinLimit, MAX(UpperLimit) AS MaxLimit FROM #A
),
RESULT AS
(
-- Get the first record corresponding with maximum upper limit
SELECT *
FROM DATA D1
WHERE NOT EXISTS
(SELECT 1
FROM DATA D2
WHERE D2.UpperLimit = D1.UpperLimit AND D2.Amount > D1.Amount)
AND D1.UpperLimit = (SELECT MaxLimit FROM MA)
-- Recursive get remain record corresponding with other upper limit
UNION ALL
SELECT D1.*
FROM RESULT R1 INNER JOIN DATA D1
ON (R1.UpperLimit > D1.UpperLimit AND R1.Id != D1.Id)
WHERE D1.UpperLimit >= (SELECT MinLimit FROM MA)
AND NOT EXISTS
(SELECT 1
FROM DATA D2
WHERE D2.UpperLimit = D1.UpperLimit AND D2.Amount > D1.Amount AND D2.Id != R1.Id)
)
SELECT DISTINCT * FROM RESULT ORDER BY UpperLimit DESC;
Demo: https://dbfiddle.uk/Y-m0K6Mk

Answer 2

Might be a bit lengthy but hopefully clear enough.
with a as
(select -- order and number rows in table A in some way
row_number() over (order by UpperLimit) as RnA,
*
from #a),
b as
(select -- order and number rows in table B in the same way
row_number() over (order by Amount) as RnB,
*
from #b),
m as
(select -- get and number all possible pairs of values from both tables considering the restriction
row_number() over (order by a.UpperLimit desc, b.Amount desc) as RnM,
*
from a
join b on
b.Amount < a.UpperLimit),
r as
(select -- use recursion to get all possible combinations of the value pairs with metrics of interest for comparison
convert(varchar(max), RnA) as ListA,
convert(varchar(max), RnB) as ListB,
RnA,
RnB,
1 as CountB,
convert(int, Amount) as SumB
from m
where RnM = 1
union all
select
r.ListA + ' ' + convert(varchar(max), m.RnA),
r.ListB + ' ' + convert(varchar(max), m.RnB),
m.RnA,
m.RnB,
r.CountB + 1,
r.SumB + convert(int, m.Amount)
from m
join r on
m.RnA < r.RnA and
m.RnB < r.RnB),
e as
(select top(1) -- select combinations of interest using metrics
ListA,
ListB
from r
order by CountB desc, SumB desc),
ea as
(select -- turn id list into table for table A
ea.Rn,
ea.Value
from e
cross apply(select row_number() over (order by (select null)) as Rn, Value from string_split(e.ListA, ' ')) as ea),
eb as
(select -- turn id list into table for table B
eb.Rn,
eb.Value
from e
cross apply(select row_number() over (order by (select null)) as Rn, Value from string_split(e.ListB, ' ')) as eb)
select -- get output table with actual values from the original tables
a.UpperLimit,
b.Amount,
b.Id
from ea
join eb on
ea.Rn = eb.Rn
join a on
ea.Value = a.RnA
join b on
eb.Value = b.RnB;

Answer 3

You can use an APPLY with a TOP 1 for this. Each row in the outer table gets only one row from the APPLY.
SELECT
*
FROM #A a
OUTER APPLY (
SELECT TOP (1) *
FROM #B b
WHERE b.Amount < a.UpperLimit
) b;
To simulate an inner-join (rather than a left-join) use CROSS APPLY.

Answer 4

This query returns very close to desired outcome.
WITH CTE AS (SELECT B.*,
ROW_NUMBER() OVER (PARTITION BY B.Value ORDER BY B.Value DESC) AS RowNum
FROM #B B),
cc as (SELECT A.Limit, CTE.*
FROM #A A
LEFT JOIN CTE ON CTE.Value < A.Limit AND CTE.RowNum = 1),
cc2 as (select *, MAX(Value) OVER ( PARTITION BY cc.Limit) as l1 from cc)
select Limit, ID, Value
from cc2
where Value = l1
This query use 3 Common Table Expressions. First sort Table B with ROW_NUMBER() function and PARTITION BY clause, second one JOIN Table A with Table B with the condition given and the third one filters the record that is in Limit on Table A and use the Limit only once.