嘿这样写怎么不行（SQL 菜鸟）？

不行的，你可以用if 代替
sql的case
是这样用的
USE pubs
GO
SELECT    'Price Category' =
      CASE
         WHEN price IS NULL THEN 'Not yet priced'
         WHEN price < 10 THEN 'Very Reasonable Title'
         WHEN price >= 10 and price < 20 THEN 'Coffee Table Title'
         ELSE 'Expensive book!'
      END,
   CAST(title AS varchar(20)) AS 'Shortened Title'
FROM titles
ORDER BY price
GO
可以看看帮助