sql storedprocedure problem

please can you help me , this's my first post , i don't know if this the correct place ,
but i need help .
i have this code in sql
SELECT
Open_and_Closed_Agencies.[U/W Code] AS agt_uw_code,
Open_and_Closed_Agencies.[Agt Code],
Dual_Assignment_Table.[U/W Code] AS dual_uw_code,
case when Dual_Assignment_Table.[U/W Code] Is Null
then @Active
else Dual_Assignment_Table.[U/W Code]
end AS Expr2

case when [expr2]=Open.
then OpenAgencies.
else case when [expr2]=Dua.[U/W Code]
then Dual_Assignment_Table.[U/W Code]
else Null
end
end
AS Expr3
FROM Open_and_Closed_Agencies
LEFT JOIN Dual_Assignment_Table ON
Open_and_Closed_Agencies.[Agt Code] = Dual_Assignment_Table.[Agt Code]
WHERE (((Open_and_Closed_Agencies.CloseDate)
Is Null Or (Open_and_Closed_Agencies.CloseDate)='1/1/1900'
Or (Open_and_Closed_Agencies.CloseDate)>GETDATE()-365))
ORDER BY Open_and_Closed_Agencies.[U/W Code];

the problem that i use expr2 in the same stored procedure , and this's not allowed , but how can i solve this problem ?
thanx .