将GROUP BY与AVG函数一起使用 - 语法错误不正确

Question

SELECT 
    AVG (CASE 
            WHEN ItemPrice LIKE '-%' 
               THEN NULL
            WHEN ItemPrice LIKE '0%' 
               THEN NULL
            ELSE CAST (ItemPrice AS FLOAT) 
         END)
GROUP BY 
    HHIncome 
FROM 
    Tacos

SQL返回以下错误：

错误：关键字“FROM”附近的语法不正确。 SQLState：S0001 ErrorCode：156

有人可以解释为什么上面的陈述本身是不正确的，但是当嵌套在下面的陈述中时可以操作吗？

SELECT  
    HHIncome AS Income_Segment, 
    COUNT(Customer) AS number_of_customers, 
    AVG (CASE 
            WHEN ItemPrice LIKE '-%' THEN NULL
            WHEN ItemPrice LIKE '0%' THEN NULL
            ELSE CAST (ItemPrice AS FLOAT) 
         END) AS avg_item_price, 
    COUNT(DISTINCT(ReceiptIDDesc)) AS number_of_orders, 
    SUM(CAST(ItemPrice AS FLOAT)) AS total_spend
FROM
    Tacos 
GROUP BY 
    HHIncome;