Having looked at the article, it looks like the 'AS' clause is in the wrong place. It should be like this, I think

SELECT a.DayCount,

a.Sales,

SUM(b.Sales) AS RunningTotal

FROM Sales a

CROSS JOIN Sales b

WHERE (b.DayCount <= a.DayCount)

GROUP BY a.DayCount,a.Sales

ORDER BY a.DayCount,a.Sales