Combinations of Products as single count - sql

Question

I need to count combinations of products within transactions differently to other products and I'm struggling with how to do this within a single select statement from SQL 2008. This would then become a data set to manipulate in Reporting Services
raw data looks like this
txn, prod, units
1, a, 2
1, c, 1
2, a, 1
2, b, 1
2, c, 1
3, a, 2
3, b, 1
4, a, 3
4, c, 2
So a+b should = one if in same trans number, however a or b should equal one if not paired. So a=1 and b=1 but a+b=1, a+b+a=2, a+b+a+b=2 given the example data here is my desired result with an explanation of why
txn 1 is 3 units -- 2a + c
txn 2 is 2 units -- (a+b) + c
txn 3 is 2 units -- (a+b) + a
txn 4 is 5 units -- 3a + 2c
My query is more complex than this and includes other aggregates so I would like to group by transaction which I can't do as I need to manipulate at a lower grain
Update Progress :
Possible solution, I've generated columns based on the products I'm measuring. This allows me to group on Txn as I am now aggregating that field. Unsure if there's a better way to do it as it does take a little while
CASE WHEN SUM(CASE WHEN Prod='a' then 1 else 0 end)-
SUM(CASE WHEN Prod='b' then 1 else 0 end)=0
THEN SUM(CASE WHEN Prod='a' then 1 else 0 end)
ELSE 0 END AS MixProd
, CASE WHEN SUM(CASE WHEN Prod='a' then 1 else 0 end)-
SUM(CASE WHEN Prod='b' then 1 else 0 end)!=0
THEN ABS(SUM(CASE WHEN Prod='a' then 1 else 0 end)-
SUM(CASE WHEN Prod='b' then 1 else 0 end))
ELSE 0 END AS NotMixProd
I will then need to sort out the current unit aggregate to remove the extras but this certainly gives me a start
Update Progress 2 :
This failed to handle 0 correctly where a or b was 0 it would still give a value for mix because a-b was not zero. I reverted to an earlier draft that I lost and expanded as per below
, CASE WHEN SUM(CASE WHEN Prod='a' then 1 else 0 end) = 0 THEN 0
WHEN SUM(CASE WHEN Prod='b' then 1 else 0 end) = 0 THEN 0
WHEN SUM(CASE WHEN Prod='a' then 1 else 0 end)-
SUM(CASE WHEN Prod='b' then 1 else 0 end)=0
THEN SUM(CASE WHEN Prod='a' then 1 else 0 end)
ELSE ABS(SUM(CASE WHEN Prod='a' then 1 else 0 end)-
SUM(CASE WHEN Prod='b' then 1 else 0 end))
END AS MixProd
, CASE WHEN SUM(CASE WHEN Prod='a' then 1 else 0 end)-
SUM(CASE WHEN Prod='b' then 1 else 0 end)!=0
THEN ABS(SUM(CASE WHEN Prod='a' then 1 else 0 end)-
SUM(CASE WHEN Prod='b' then 1 else 0 end))
ELSE 0 END AS NotMixProd

Answer 1

UPDATE: This should work in SQL Server 2008 (based on LAG solution from here).
Here is the demo: http://rextester.com/GNI23706
WITH CTE AS
(
select txn, prod, units,
row_number() over (partition by txn order by prod) rn,
(row_number() over (partition by txn order by prod))/2 rndiv2,
(row_number() over (partition by txn order by prod)+1)/2 rnplus1div2,
count(*) over (partition by txn) partitioncount
from test_data
)
select
txn,
sum(case when prev_prod = 'a' and prod = 'b' and prev_units >= units then 0
when prev_prod = 'a' and prod = 'b' and prev_units < units then units - prev_units
else units
end) units
from
(
select
txn,
prod,
units,
CASE WHEN rn%2=1
THEN MAX(CASE WHEN rn%2=0 THEN prod END) OVER (PARTITION BY txn,rndiv2)
ELSE MAX(CASE WHEN rn%2=1 THEN prod END) OVER (PARTITION BY txn,rnplus1div2)
END AS prev_prod,
CASE WHEN rn%2=1
THEN MAX(CASE WHEN rn%2=0 THEN units END) OVER (PARTITION BY txn,rndiv2)
ELSE MAX(CASE WHEN rn%2=1 THEN units END) OVER (PARTITION BY txn,rnplus1div2)
END AS prev_units
from cte
) temp
group by txn
For SQL Server 2012+, use LAG:
select
txn,
sum(
case when prev_prod = 'a' and prod = 'b' and prev_units >= units then 0
when prev_prod = 'a' and prod = 'b' and prev_units < units then units - prev_units
else units
end) units
from
(
select
txn,
prod,
units,
lag(prod) over (partition by txn order by prod) prev_prod,
lag(units) over (partition by txn order by prod) prev_units
from test_data
) temp
group by txn

Answer 2

I decided in the end that a temp table was the best way to go, because I couldn't group on a collation. So I eventually tweaked the code above as it was failing to pick up the spare items correctly
SUM(Units) AS OldUnits
SUM(Units) -
(CASE WHEN
SUM(CASE WHEN Prod='a' THEN 1 ELSE 0 END) = 0 THEN 0 WHEN
SUM(CASE WHEN Prod='b' THEN 1 ELSE 0 END) = 0 THEN 0 WHEN
SUM(CASE WHEN Prod='a' THEN 1 ELSE 0 END) -
SUM(CASE WHEN Prod='b' THEN 1 ELSE 0 END) = 0 THEN
SUM(CASE WHEN Prod='a' THEN 1 ELSE 0 END) WHEN
(SUM(CASE WHEN Prod='a' THEN 1 ELSE 0 END) -
SUM(CASE WHEN Prod='b' THEN 1 ELSE 0 END)) < 0 THEN
SUM(CASE WHEN Prod='a' THEN 1 ELSE 0 END) ELSE
SUM(CASE WHEN Prod='b' THEN 1 ELSE 0 END) END) AS NewUnits
This was stored in a temptable that I could then collate on Trans as the next step. Works fine for my purposes and helped me overcome a mild irrational fear I have of temptables