I think Doug’s (Duffanator) point is that it’s not going to be a simple query. 
Subqueries are necessary to grab an aggregate (SUM, AVG, etc.) and go back and grab the second to last value.
Here’s a table I created (SQL Server 2000) called Table1
[code]________________
| ndx | sample |
| 1 | 123.4 |
| 2 | 234.5 |
| 3 | 345.6 |
| 4 | 432.1 |
----------------[/code]
To get the average of the last two values (that’s what i/2[/i]) really is:
SELECT AVG(sample) ## SUM(sample)/2 works also ##
FROM (SELECT TOP 2 *
FROM Table1
ORDER BY ndx DESC) t
To get the second to last value
SELECT TOP 1 sample
FROM (SELECT TOP 2 *
FROM Table1
ORDER BY ndx DESC) t
ORDER BY ndx
So that’s two queries. Now you need a third one to put it all together subtracting the second one (and get the absolute value-- I didn’t forget! 
).
SELECT ABS
((SELECT AVG(sample)
FROM (SELECT TOP 2 *
FROM Table1
ORDER BY ndx DESC) t) -
(SELECT TOP 1 sample AS Expr1
FROM (SELECT TOP 2 *
FROM Table1
ORDER BY ndx DESC) t
ORDER BY ndx)
Sometimes the trick is knowing when to use SQL or when it’s better to write a script:
[code]query=“SELECT TOP 2 * FROM Table1 ORDER BY ndx DESC”
result=system.db.runQuery(query, “database”)
b=result[0][“sample”] #remember, they came in reverse order
a=result[1][“sample”]
print abs((a+b)/2-a)[/code]
Hope this helps!