How do I use alias in where clause?

Question

Possible Duplicate:
Referring to a Column Alias in a WHERE Clause

   SELECT
Trade.TradeId, 
Isnull(Securities.SecurityType,'Other') SecurityType, 
TableName,
CASE 
WHEN 
SecurityTrade.SecurityId IS NOT NULL  
THEN 
SecurityTrade.SecurityId
ELSE 
Trade.SecurityId
END AS PricingSecurityID,
sum(Trade.Quantity)OVER(Partition by Securities.SecurityType, SecurityTrade.SecurityId,Trade.Price, Buy,Long ) as sumQuantity,
--added porfolio id for Getsumofqantity
Trade.PortfolioId,

Trade.Price,
case
when (Buy = 1 and Long = 1) then 1
when (Buy = 0 and Long = 0) then 1
else 0
end Position
from
Fireball_Reporting..Trade

where porfolioid =5 and Position =1   

i want to use Position =1 in my where clause which is an alias of case

case
when (Buy = 1 and Long = 1) then 1
when (Buy = 0 and Long = 0) then 1
else 0
end Position

How can I use it in where clause?

I tried zo directly use that CASE statement in where clause, but failed.

WHERE Trade.SecurityId = @SecurityId AND PortfolioId = @GHPortfolioID AND
                (case when (Buy = 1 and Long = 1) then 1 when (Buy = 0 and Long = 0) then 1 else 0 end Position = 1)

Answer 1

The SQL-Server docs says:

column_alias can be used in an ORDER BY clause, but it cannot be used in a WHERE, GROUP BY, or HAVING clause.

Similar in the MySQL doc it says:

Standard SQL disallows references to column aliases in a WHERE clause. This restriction is imposed because when the WHERE clause is evaluated, the column value may not yet have been determined.

In MySQL you can at least reuse aliases in the SELECT clause