>>As far as I know, vfp always stops evaluating the right remainder of a logical expression if the left part evaluates to an logical value that cannot be changed anymore by the right remainder. For example, in the case of IF A OR B, the B is no longer evaluated if A evaluates to True. In a same vein, in the case of IF A=0 or BB(), the function BB is not visited if A=0.
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>>But now I am confronted with a case in a select-sql WHERE expression, where the part to the right of the OR is visited anyhow, notwithstanding the result of the left expression.

WHERE ( MOD(A,1000)#0 OR THERMO(A) )

>>
>>Does this sound familiar to anyone here?
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>I suppose that within a SQL command different rules apply, what with Rushmore Optimization. For example, in one particular case Visual FoxPro might decide that it is more efficient to evaluate the right-hand member first, because it is Rushmore-Optimizable. In any case, I wouldn't count on the right-hand expression not being evaluated within a SQL command (on the other hand, I would trust VFP to do shortcut boolean evaluation in most other cases).
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>If shortcut boolean evaluation is important in this case, you might move the entire combined expression to a UDF, or change it to an IIF().

Great, your tip to use IIF did help!

WHERE IIF(MOD(A,1000)=0,THERMO(A),.T.)

And that's the most important thing for me now. Remains the quest whether the rule is not valid inside a SQL command. If that is indeed the case, then it should be documented, at the least.