Using Aliases in Where Clause or an Alternative Option?

Question

How do I get this to work, it works without the Where Clause, otherwise with the Where clause, i get the obvious error, but that's basically what needs to be done, anyone know how to approach this?

select ID, 
       Name,
       case T.N 
         when 1 then City1
         when 2 then City2
         when 3 then City3
       end as City,
       case T.N 
         when 1 then State1
         when 2 then State2
         when 3 then State3
       end as State
from YourTable
  cross join (values(1),(2),(3)) as T(N)

    Where City is NOT Null

Answer 1

You can't use the alias in the WHERE clause. Either repeat the expression (messy) or else put your SELECT in a subquery and then put the WHERE clause in the outer query:

SELECT Id, Name, City, State
FROM
(
     SELECT
         ID, 
         Name,
         CASE T.N 
             WHEN 1 THEN City1
             WHEN 2 THEN City2
             WHEN 3 THEN City3
         END AS City,
         CASE T.N 
             WHEN 1 THEN State1
             WHEN 2 THEN State2
             WHEN 3 THEN State3
         END AS State
     FROM YourTable
     CROSS JOIN (VALUES(1),(2),(3)) AS T(N)
) T1
WHERE City IS NOT NULL

Answer 2

You can't use an alias (from SELECT clause) in WHERE clause because the logical processing order(section: Logical Processing Order of the SELECT statement) is WHERE and then SELECT:

FROM    
ON
JOIN
WHERE <--
GROUP BY
WITH CUBE or WITH ROLLUP
HAVING
SELECT <--
DISTINCT
ORDER BY <--
TOP

But, you can use an alias in ORDER BY:

SELECT  h.SalesOrderID, YEAR(h.OrderDate) OrderYear
FROM    Sales.SalesOrderHeader h
ORDER BY OrderYear;

Solutions: see the solutions presented by Mark Byers.

Tibor Karaszi: Why can't we have column alias in ORDER BY?