Crystal Report HELP!

Can I make a Query with in a Query...? How?
My Query...

SELECT Calls.*, CallId AS Expr1, Duration AS Mins,
Duration AS Secs, Duration AS Hrs,
'01/01/2000' AS StartDate, DateDiff('y', StartDate,
`Date`) AS DaysDiff
FROM Calls
WHERE (CallId <> ' ') AND (DateDiff('y', StartDate, `Date`)
= 623)

The Error is...

No Value Given for One Or More Required Paramets


This is because DaysDiff Parameter has no value until the query has fully executed.... so my problem is to filter all records whoes DaysDiff equal to example 623.... I hope I you understand.....