How to refer to computed columns from other computed columns?

[Matthew Wilson <matt@xxxxxxxxxx>]

I'm converting some procedural code to SQL as an experiment.  Here's the
pseudocode:

    c = a - b
    if c < 0 then d = 'no'
    else d = 'yes'

In SQL, I've got this:

    select a, b, a - b as c,
    case when a - b < 0 then 'no'
    else 'yes'
    end as d

    from foo;

This is a trivial example, but you can see how I calculate a - b two
separate times.

In reality, I have much nastier calculations and they happen more than
just twice.

I'm looking for an elegant solution for this puzzle.  I don't want to
repeat that a - b part over and over because I likely will need to
change how c gets defined and I don't want to have to change more than
one place in the code.

All I can come up with so far is to use a view and then another view on
top of that one:

    create view v1 as
    select a, b, a - b as c
    from foo;

    create view v2 as
    select a, b, c,
    case when c < 0 then 'no'
    else 'yes'
    end as d
    from v1;

This is better than the first solution because c is only defined in a
single place.  Is this the best possible solution?

Thanks for the help.

Matt


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