Two ways:
Original: SELECT (Quantity * Price) / (Quantity * UnitPrice) AS [the average] FROM ## Table error: encountered to divide by zero error
1, where the divisor is determined in the denominator is 0 (a 'NULLIF' function)
SELECT (Quantity * Price) / NULLIF ((Quantity * UnitPrice), 0) AS [average] FROM ## table
2, where the divisor is determined in the denominator is 0 (a 'CASE WHEN' function)
SELECT (CASE WHEN Quantity*UnitPrice=0 THEN 0 ELSE (Quantity*Price)/(Quantity*UnitPrice)) AS [平均值] FROM ##表