No, the SELECT statement is designed that way.
You can achieve what you want like this (its not good practise)
IF NOT EXISTS (SELECT * FROM products
WHERE productname = 'bob')
SELECT 0 AS price;
SELECT price FROM products
WHERE productname = 'bob';
However - how do you know if the productname 'bob' does exist, but has a price of 0? - you can't tell from the results which condition gave the result, so its not a reliable test for the existance of productname 'bob'