SQL Server Include user function result into select query - sql

Question

I have this stored function
function GetPrevReading(
#utility int,
#asofdate datetime
) returns decimal(10,5)
This function returns the previous meter reading from the table with the following fieds:
utility - int
date - datetime
reading - numeric(18,4)
When I use select on this table I want to set a date as a parameter and get this from the table:
Utility Previous Reading
(distinct) GetPrevReading(utility from query, #date from parameter)
I want the function GetPrevReading to take parameter 'utility' from the current row.
Is it possible to accompish this with a query or should I make a stored procedure?
For example, this is the table:
Utility Date Reading
1 2013-10-1 105.6
1 2013-11-1 123.72
2 2013-10-1 226.1
2 2013-10-1 238.18
Now, if I set parameter #date to 2013-10-29 I should get this result:
Utility PreviousReading
1 105.6
2 226.1
Here, my function should get #utility=1 and #asofdate='2013-10-29' on the first row and #utility=2 and #asofdate='2013-10-29' on the second one.

Answer 1

Try this out. I fixed some inconsistencies in your data types, and assumed that your last line of sample data really should have had 2013-11-01 as the date. Also, the way that the function is written, it's not getting the previous reading, but the reading on that date.
CREATE TABLE MyTable (
Utility Int,
Date Date,
Reading Decimal(10,5)
);
INSERT INTO MyTable (Utility, Date, Reading)
VALUES
(1,'2013-10-01', 105.60),
(1,'2013-11-01', 123.72),
(2,'2013-10-01', 226.10),
(2,'2013-11-01', 238.18);
CREATE FUNCTION dbo.GetPrevReading(
#utility int,
#asofdate datetime
)
RETURNS Decimal(10,5)
AS
BEGIN
RETURN (
SELECT TOP 1 Reading
FROM MyTable
WHERE Utility = #Utility
AND Date = #asofdate
ORDER BY Date DESC
)
END;
SELECT
Utility
,Date
,dbo.GetPrevReading(Utility, Date)
FROM (
SELECT Utility, Max(Date) Date
FROM MyTable
WHERE Date < '2013-10-29'
GROUP BY Utility
) x;

Answer 2

Am I understanding the question; the function returns for this call
GetPrevReading( 1,2013-10-29)
Returns
1, 105.6
2, 226.1
And you want to join between the function and its results and the underlying table? You can do this in SQL 2005 + using the Apply join
Select
…
From tblUtilityReadings
Cross Apply GetPrevReading(tblUtilityReadings.utility, #date)