How to solve Divide by Zero exception in SQL stored procedure? - sql-server-2005

Question

When I am executing my stored procedure it is throwing Divide By zero Exception. Because In my database there is no data for some months. How can I solve this?
Below is the my query.
#diffNSVY FLOAT = 0 --I have declared #diffNSVY as optional parameter.
SET #diffNSVY = CASE
WHEN (select top 1 NSV from #temp where rn = 1) < 0 THEN 0
ELSE (((select top 1 NSV from #temp where descrn = 1) - (select top 1 NSV from #temp where rn = 1))*1.0)/(select top 1 NSV from #temp where rn = 1)
END
I am inserting a result set into #temp table. NSV is a column name. rn is rownumber.descrn is also row number in decreasing order.
How can I modify my query?
Please reply.
Regards,
NSJ

Answer 1

First, I would rebuild your script so I didn't need to repeat complex expressions (the subselects, to be precise) more than once.
If possible, use SELECT instead of SET, like this:
SELECT #diffNSVY = CASE
WHEN rn.NSV < 0 THEN 0
ELSE (descrn.NSV - rn.NSV) * 1.0 / rn.NSV /* extra '()' are removed as unneeded */
FROM
(select top 1 NSV from #temp where rn = 1) AS rn,
(select top 1 NSV from #temp where descrn = 1) AS descrn
Next, ask yourself, what the result should be in case of division by zero. Should it be zero as well? Then the next optimisation step would be simply this:
SELECT #diffNSVY = CASE
WHEN rn.NSV <= 0 THEN 0 /* changed '<' to '<=' to account for division by zero */
ELSE (descrn.NSV - rn.NSV) * 1.0 / rn.NSV
FROM
(select top 1 NSV from #temp where rn = 1) AS rn,
(select top 1 NSV from #temp where descrn = 1) AS descrn
But if you wish the result to become undefined (NULL) so you process it later, here's how you can achieve this:
SELECT #diffNSVY = CASE
WHEN rn.NSV < 0 THEN 0
ELSE (descrn.NSV - rn.NSV) * 1.0 / CASE rn.NSV WHEN 0 THEN NULL ELSE rn.NSV END
FROM
(select top 1 NSV from #temp where rn = 1) AS rn,
(select top 1 NSV from #temp where descrn = 1) AS descrn
Generally, I find this pattern useful when I need to secure myself from divide-by-zero errors. I often use it like this:
...ISNULL(some_expression / CASE #Value WHEN 0 THEN NULL ELSE #Value END, 0)...
Sometimes I use it without ISNULL in which case I process the possible NULL result later using some more sophisticated logic.
EDIT: Oh, and one more thing: with SELECT you can have several assignments at once, like this:
SELECT
#Var1 = expression1,
#Var2 = expression2,
...
Could this possibly help you to simplify your query too?

Answer 2

That expression of yours is very unclear and hard to understand, and you're selecting the same value several times which is totally unnecessary - so my recommendation would be:
try to first determine all the bits and pieces that might go into your calcuation - put the results of those select top 1 .... queries into variables
then check before you divide by zero, and if you divisor would be zero, you need to think of another solution / another value to use instead...
Your problem is this: you're currently only checking for your one value being < 0 and then your return 0 - otherwise (including when that value is = 0) you return an expression which is a division by exactly that value.... You need to add one more special case: if that value is = 0, you cannot use your expression since that results in the divide by zero exception - you need to return some other value in this case!
So your code would be something like:
DECLARE #diffNSVY FLOAT = 0 --I have declared #diffNSVY as optional parameter.
DECLARE #RNValue INT
SET #RNValue = (SELECT TOP 1 NSV FROM #temp WHERE rn = 1)
DECLARE #DescRNValue INT
SET #DescRNValue = (SELECT TOP 1 NSV FROM #temp WHERE descrn = 1)
SET #diffNSVY =
CASE
WHEN #RNValue < 0 THEN 0
WHEN #RNValue = 0 THEN ....... <-- you need to define a value here! CAnnot divide by #RNValue if it's ZERO !
ELSE ((#DescRNValue - #RNValue) * 1.0) / #RNValue
END

Answer 3

Maybe you need a '<=0' not '<0' in your if clause? You should also probably make sure there's data in your temp table that meets the rn = 1 criteria, otherwise the selection will return null. If all else fails Sql2005 has try catch blocks, so you can catch the divide by zero exception.