sub-selects

Is there a way to make a query more efficient by executing a sub-select only once?

In a query such as:

SELECT a, (select b from c where d = e limit 1), npoints( (select b from c where d = e limit 1) )

FROM f

WHERE isValid( (select b from c where d = e limit 1) );

I do the same sub-select 3 times in the query. I tried the following:

SELECT a, (select b from c where d = e limit 1) AS g, npoints( g )

FROM f

WHERE isValid( g );

But this gave an error regarding "column 'g' does not exist". How can I avoid making the same sub-select 3 times?

Mark