SQL - Define a column and use it in the same query

Question

Can I define a new column and use it in the same query?

For example,

Select 
    student, age + 10 as trueage, 
    case when trueage < 25 then 'False' else 'True' end
From 
    class

Thank you in advance for helping!

Answer 1

CROSS APPLY would be handy here. Here is a nice article showing many uses of it.

Select
    student
    ,trueage
    ,case when trueage < 25 then 'False' else 'True' end
From
    class
    CROSS APPLY
    (
        SELECT age + 10 as trueage
    ) AS CA

It is mostly useful when you have complex formulas with several levels, something like this:

Select
    student
    ,trueage
    ,AgeFlag
From
    class
    CROSS APPLY
    (
        SELECT age + 10 as trueage
    ) AS CA1
    CROSS APPLY
    (
        SELECT case when trueage < 25 then 'False' else 'True' end AS AgeFlag
    ) AS CA2

Answer 2

If you consider using a subquery "same query" then yes.

select student,
       trueAge,
       case when trueAge < 25 then 'false' else 'true' end as someColumn
from ( 
    select student, age + 10 as trueAge
    from table
) thingy

There are likely other ways to do this as well, this is only one of them.