SQL Server 2008 - need help on a antithetical query - sql

Question

I want to find out meter reading for given transaction day. In some cases there won’t be any meter reading and would like to see a meter reading for previous day.
Sample data set follows. I am using SQL Server 2008
declare #meter table (UnitID int, reading_Date date,reading int)
declare #Transactions table (Transactions_ID int,UnitID int,Transactions_date date)
insert into #meter (UnitID,reading_Date,reading ) values
(1,'1/1/2014',1000),
(1,'2/1/2014',1010),
(1,'3/1/2014',1020),
(2,'1/1/2014',1001),
(3,'1/1/2014',1002);
insert into #Transactions(Transactions_ID,UnitID,Transactions_date) values
(1,1,'1/1/2014'),
(2,1,'2/1/2014'),
(3,1,'3/1/2014'),
(4,1,'4/1/2014'),
(5,2,'1/1/2014'),
(6,2,'3/1/2014'),
(7,3,'4/1/2014');
select * from #meter;
select * from #Transactions;
I expect to get following output
Transactions
Transactions_ID UnitID Transactions_date reading
1 1 1/1/2014 1000
2 1 2/1/2014 1010
3 1 3/1/2014 1020
4 1 4/1/2014 1020
5 2 1/1/2014 1001
6 2 3/1/2014 1001
7 3 4/1/2014 1002

Answer 1

Your SQL Query to get your desired out put will as following:
SELECT Transactions_ID, T.UnitID, Transactions_date
, (CASE WHEN ISNULL(M.reading,'') = '' THEN
(
SELECT MAX(Reading) FROM #meter AS A
JOIN #Transactions AS B ON A.UnitID=B.UnitID AND A.UnitID=T.UnitID
)
ELSE M.reading END) AS Reading
FROM #meter AS M
RIGHT OUTER JOIN #Transactions AS T ON T.UnitID=M.UnitID
AND T.Transactions_date=M.reading_Date

Answer 2

I can think of two ways to approach this - neither of them are ideal.
The first (and slightly better) way would be to create a SQL Function that took the Transactions_date as a parameter and returned the reading for Max(Reading_date) where reading_date <= transactions_date. You could then use this function in a select statement against the Transactions table.
The other approach would be to use a cursor to iterate through the transactions table and use the same logic as above where you return the reading for Max(Reading_date) where reading_date <= transactions_date.

Answer 3

Try the below query:
Please find the result of the same in SQLFiddle
select a.Transactions_ID, a.UnitID, a.Transactions_date,
case when b.reading IS NULL then c.rd else b.reading end as reading
from
Transactions a
left outer join
meter b
on a.UnitID = b.UnitID
and a.Transactions_date = b.reading_Date
inner join
(
select UnitID,max(reading) as rd
from meter
group by UnitID
) as C
on a.UnitID = c.UnitID