This sorts everything by column1 (descending) first, and then by column2 (ascending, which is the default) whenever the column1 fields for two or more rows are equal.
You can use multiple ordering on multiple condition,
ORDER BY
(CASE
WHEN @AlphabetBy = 2 THEN [Drug Name]
END) ASC,
CASE
WHEN @TopBy = 1 THEN [Rx Count]
WHEN @TopBy = 2 THEN [Cost]
WHEN @TopBy = 3 THEN [Revenue]
END DESC
