本人入坑不久，公司安排我改模型，时间很紧，自己去学习改来不及，所以请教各位大佬，原函数是通过计算得到yongneng，现在有了个需求，优先引用B表中的yongneng字段，如果为空，再通过原函数计算得到yongneng，这里好像要用case when，各位大佬们，应该怎么改？如下是原函数：
isnull(
CAST (
(
0.7143 * CAST (isnull(j008.TF002, 0) AS NUMERIC (18, 2))
+ 1.229 / 10000 * CAST (isnull(j002.TF001, 0) AS NUMERIC (18, 2))
+ 0.0341 * CAST (isnull(j008.TF004, 0) AS NUMERIC (18, 2))
+ 13.3 * CAST (isnull(j003.TF001, 0) AS NUMERIC (18, 2))
+ 1.4714 * CAST (isnull(j008.TF004, 0) AS NUMERIC (18, 2))
+ 1.4571 * CAST (isnull(j008.TF003, 0) AS NUMERIC (18, 2))
) AS NUMERIC (18, 2)
),
0
) AS yongneng
case when yongneng is null then 计算公式 else yongneng end as 一个字段