I think VFP will evaluate it twice. I'm not sure about SQL Server. What problem does it create for you if expression is evaluated more than once?
Anyway, if you don't want it to be evaluated twice for sure, use derived table

* VFP
*SELECT IIF(FLD1 - FLD2 < 0, 0, FLD1 - FLD2) as NEWCOLUMN from MyTable
SELECT IIF(days < 0, 0, days) as newcolumn ; 
	FROM ( SELECT fld1-fld2 AS days FROM MyTable) dt1

-- SQL Server
*SELECT CASE WHEN FLD1 - FLD2 < 0 THEN 0 ELSE FLD1 - FLD2 END as NEWCOLUMN from MyTable
SELECT (CASE WHEN days < 0 THEN 0 ELSE days END) as newcolumn  
	FROM ( SELECT fld1-fld2 AS days FROM MyTable) dt1

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>Do you know or do you think that SQL Server (and VFP too) evaluate the expression FLD1 - FLD2 twice or the engines are smart enough to only do it once? I am just curious if I should get the cursor with just the Days difference column and then process this cursor in VFP for negative values.