You can use CASE function to check for nSd=0 and return whatever value you want. For example

CASE WHEN nSd<>0 THEN (a.nResult - nMean) / nSd ELSE NULL END

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>I have the following query that performs some statistical calculations:
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<snip>
The problem is with the nSdi calculations. The formula for this field is (a.nResult - nMean) / nSd. When try the execute the query I am getting the Divide by zero error encountered error mesage.