Setting a variable in a SQL WHERE clause to be used in SELECT

Question

I'm using Transact-SQL with Microsoft SQL Server, and we have a query that looks like this:

SELECT Cast( Cast ( Cast(XMLBlob as XML).query(N'//continent/forest/tree/age/Text()') as nvarchar) as bigint), 
AnotherField
FROM [MyDB].[dbo].[mytable]
WHERE Cast( Cast ( Cast(XMLBlob as XML).query(N'//continent/forest/tree/age/Text()') as nvarchar) as bigint) 
between 10 and 100 

The XML cast is an expensive operation, and since it's used in both the WHERE and SELECT, it seems like I should be able to save it away as a variable in the WHERE (which, by order of operations, is evaluated before the SELECT), and use it in the SELECT instead of having to cast again. Is this possible?

Answer 1

You could use an inner query where you retrieve the XML value. Then outside the inner query you both return that bigint value and filter the values you want:

SELECT innerTable.Age, innerTable.AnotherField
FROM (
    SELECT Cast( Cast ( Cast(XMLBlob as 
    XML).query(N'//continent/forest/tree/age/Text()') as nvarchar) as bigint) AS Age, 
    AnotherField
    FROM [MyDB].[dbo].[mytable]
) AS innerTable
WHERE innerTable.Age between 10 and 100 

By the way... why do you need a bigint to store Age? If you are storing years looks like overkill, even for those trees that live thousands of years :)