如何在sql中的unpivot中执行空检查

我在sql server上使用以下sql查询

select distinct  [Value],Label,Id from(
select distinct [Name], Code, I as Id,I + ':' + [Value] as label, [Value]
from [test].[dbo].[emp]
unpivot
(
  [Value]
  for I in (product, model)
) as dataTable) as t

我想要的是，如果unpivot语句中的任何[Value]为null，它应该向嵌套的select语句返回“unknown”

我如何做到这一点

更新-

//this is wrong sql. Just want to show what is required
select distinct  [Value],Label,Id from(
select distinct [Name], Code, coalesce(I as Id,'unknown'),coalesce(I,'unknown') + ':' + [Value] as label, coalesce([Value],'unknown')
from [test].[dbo].[emp]
unpivot
(
  [Value]
  for I in (product, model)
) as dataTable) as t

---

您似乎在寻找联合：

我建议将问题写为：

我不确定您真正想要什么，但我建议使用apply编写查询

---

这似乎简单得多，而且性能可能也更好。

样本数据和期望的结果会有所帮助。

from (select distinct [Name], Code, I as Id,
             I + ':' + coalesce([Value], 'unknown') as label,
            [Value]
      from [test].[dbo].[emp]
     ) e

select distinct v.id, (v.id + ':' + coalesce(v.[Value], 'unknown')) as label,
       coalesce(v.[Value], 'unknown') as value
from [test].[dbo].[emp] e cross apply
     (values (e.product, 'product'), (e.model, 'model')
     ) v(value, id);