SQL如何求和if值

嗨，我试着需要求和值，如果值超过20，则应该写20，否则应该是数字

我不知道我做错了什么：

SELECT
    ta6.product_product_id AS ID,
    ta6.product_manufacture_code AS Code,
    SUM(CASE WHEN ta3.stock_quantity >= 20 THEN 20 ELSE ta3.stock_quantity END) AS Quantity,
    ta4.price_value AS Price,
    ta5.Attribute_Value AS Sizer,
    ta6.Procut_product_name AS Name,
    ta7.Attribute_Value AS produce
FROM
    product ta6
LEFT JOIN stock ta3 ON
    ta3.stock_id = ta6.product_id
LEFT JOIN price ta4 ON
    ta4.price_id = ta6.product_id
    AND ta4.price_type = 2
LEFT JOIN Attributes ta5 ON
    ta5.Attributes_product_Id = ta6.product_id
LEFT JOIN Attributes ta7 ON
    ta7.Attributes_product_Id = ta6.product_id
WHERE
    ( ta3.stock_wharehouse_id = 1
    AND ta5.Attrib_id = 54
    AND ta7.Attrib_id = 25 )
GROUP BY
    ta6.product_manufacture_code,
    ta6.product_product_id
    ta4.price_value,
    ta5.Attribute_Value,
    ta6.product_product_name ,
    ta7.Attribute_Value;

表库存数量如下所示：

怎么把它加起来？？？

像这样

CASE WHEN SUM(ta3.stock_quantity) >= 20 
     THEN 20 
     ELSE SUM(ta3.stock_quantity) 
     END AS Quantity