别名被标记为下划线为错误

Question

查询为：

SELECT prd.*, 
       CASE 
         WHEN pr.type = 1 THEN Round(prd.price - ( prd.price * pr.value / 100 ), 
                               2) 
         WHEN pr.type = 2 THEN Round(prd.price - pr.value, 2) 
         ELSE Round(prd.price, 2) 
       END promo_price 
FROM   product AS prd 
       LEFT JOIN promotion_product AS prPrd 
              ON prd.id = prPrd.main_product_id 
       LEFT JOIN promotion AS pr 
              ON prPrd.promo_id = pr.id 
WHERE  prd.category_id = 8 
AND promo_price BETWEEN 46.15 AND 75.60 
ORDER  BY sort ASC 

我收到错误消息：Unknown column 'promo_price' in 'where clause'。这是为什么 ？我尝试用as加上括号，用as加上括号。我以前使用过这样的语法。我现在的错误在哪里？