Question

I am trying to execute the below query.

I am getting the same column, but for two consecutive days. The value I am inserting is day2.Value – day1.Value.

When tab2 does not contain a value (day2), the value to be inserted should just be 0 – day1.Value

The part not working is this -day1.Value when I have data for tab1 but not tab2. Instead I just get a null?

UPDATE tab2
SET Change = ISNULL(tab2.Value ,0) - ISNULL(tab1.Value ,0)
FROM Table_Name tab1
LEFT OUTER JOIN Table_Name  tab2 
    ON tab2.COBDate = '2012-5-18' 
    AND tab1.FileName = tab2.FileName 
    AND tab1.ID = tab2.ID  
    AND tab1.ScenarioID = tab2.ScenarioID
WHERE tab1.COBDate = '2012-5-17' 
    AND tab1.FileName = 'GBP.csv'

Answer 1

Try using COALESCE()?

UPDATE tab2
SET Change = COALESCE(tab2.Value ,0) - COALESCE(tab1.Value ,0)
FROM Table_Name tab1
LEFT OUTER JOIN 
Table_Name  tab2 
ON tab2.COBDate = '2012-5-18' AND 
tab1.FileName = tab2.FileName AND 
tab1.ID = tab2.ID  AND 
tab1.ScenarioID = tab2.ScenarioID
WHERE tab1.COBDate = '2012-5-17' AND tab1.FileName = 'GBP.csv'