Question

I have a table (in SQL Server 2005) of daily weather data for a single location which includes these columns:

LogDate                 DATETIME  
HighTemp                INT  
Temp6MonthHighAverage   INT  

LogDate and HighTemp have data. HighTemp6MonthAverage will be populated with, as the name suggests, the average high temperature for the 6 months ending in LogDate.

There are similar requirements for LowTemp, as well as humidity and several other items, for data spanning decades.

I find myself thinking in circles. Can I derive this average for each row in an UPDATE statement using set operations, or do I need to implement a solution with cursors? I will appreciate any suggestions.

Answer 1

-- select 
select HighTemp, LogDate,(select AVG(HighTemp) 
                          from tbl where 
                          DATEDIFF(MONTH, LogDate, t1.LogDate) between 0 and 6)
from tbl t1

-- update
update t1 set Temp6MonthHighAverage = (select AVG(HighTemp) 
                          from tbl where 
                          DATEDIFF(MONTH, LogDate, t1.LogDate) between 0 and 6)
from tbl t1