T-SQL (SS2K8) :: Obtain A Record Set That Returns Just First Occurrence Of ItemNo Field

Here is a CTE query

With mstrTable(ItemNo, Sales)
as (
Query1
Union All
Query2
)
Select Row_Number () Over(Partition by ItemNo order by ItemNo)as RowNo, ItemNo, Sales
From mstrTable
order by ItemNo

The results from Query1 and Query2 overlap sometimes.

The result set looks like:

1 Item1 10000
2 Item1 10000
1 Item2 20000
1 Item3 30000

I only want the first occurrence of each item. The desired result set is:

1 Item1 10000
1 Item2 20000
1 Item3 30000

I am not able to add a "Where RowNo = 1" to the query. SQL returns an "invalid field name".How would I obtain a record set that returns just the first occurrence of the ItemNo field?