求助ing,sql语句报错,

select p.part_no as po ,p.in_Client as ref_id,convert(char(10), h.ok_date,20) as date_io,
  datediff(day,h.ok_date,getdate() ) as days ,case when datediff(day,h.ok_date,getdate() ) then p.pn_in_stock else 0 end as Over_days From msh_in_part p,msh_in_head h 
  where p.pro_id=h.pro_id and h.in_job_id=p.in_job_id and h.in_ok= 'Y' 
  and p.pn_in_stock > 0 and datediff(day,h.ok_date,getdate() )

执行之后报‘消息 156，级别 15，状态 1，第 1 行
在关键字 'then' 附近有语法错误。’
各位帮忙看看了，case when .....then语句到底哪里用错 了，谢谢了...

in_stock else 0 end as ?
else（）？这是什么

中英文是否正确，还有把逗号去掉测试测试会出什么错，括号也可以的！ 只要你保证你写的没问题！