表 字段  ID date1 date2 date3 day

如果date3为空，date2不为空， day= ((Now-date2)+(date2-date1))/365;
如果date3不为空，date2为空， day= (date3-date1)/365;
sql怎么写

日期是不能直接相加减的，你必须指定是什么相加减，是日期，月份还是年份之类的，看你的题意，应该是日期之间的换算吧，以下是SQL

select case
        when date3 is null and date2 is not null then (datediff(dd,date2,getDate()))+(datediff(dd,date1,date2))/365
        when date3 is not null and date2 is null then (datediff(dd,date1,date3))/365
        end as day
from 表

select case when date3 is null then (case when date2 is not null then ((Now-date2)+(date2-date1))/365)
           else (case when data2 is not null then (date3-date1)/365) end as day from table where.....