First, what you need to do is think of each test as returning a true/false result, and then format the tests as follows:

WHERE 1 =
CASE
WHEN (Param <> 0) THEN 1
WHEN (Some OtherTest) THEN 1
WHEN (Yet another Test) THEN 1
ELSE 0
END


Sean

Thanks Sean.  I think I have understood the concept.

This gives me my iID = 72 record:
    WHERE 1 = CASE
        WHEN 72 > 0 AND tDevelopment.iID = 72 THEN 1
        WHEN 72 = 0 AND (CAST(tDevelopment.tDT AS DATE) BETWEEN (SELECT US1.dValue
                    FROM dSetValue('Current Year', 'From') US1)
                        AND (SELECT US2.dValue
                    FROM dSetValue('Current Year', 'To') US2)) THEN 1
        ELSE 0 END

This one now give me all records for the given date range:
    WHERE 1 = CASE
        WHEN 0 > 0 AND tDevelopment.iID = 0 THEN 1
        WHEN 0 = 0 AND (CAST(tDevelopment.tDT AS DATE) BETWEEN (SELECT US1.dValue
                    FROM dSetValue('Current Year', 'From') US1)
                        AND (SELECT US2.dValue
                    FROM dSetValue('Current Year', 'To') US2)) THEN 1
        ELSE 0 END

Kind regards
Bhavbhuti