Select p.ProductID,
Stuff((Select ','+Cast(ImageID as varchar(10))
From @ProductImages i
Where p.ProductID=i.ProductId
For XML PATH('')
),1,1,''
) as ImageList
From @Products p
Where p.ProductID in (Select ProductID From @ProductImages)
OutPut:
ProductID ImageList
--------- ---------
1 1,2,3
2 4,5
3 1
4 3,5