this is the one of the way to handle those error.

DECLARE @sortby VARCHAR(10)

SET @sortby='String2'

SELECT String1, String2, Date1

FROM (

SELECT 'StringValue1', '2', CONVERT(datetime,'1 July 2010 00:01:00')

UNION ALL

SELECT 'StringValue2', '1', CONVERT(datetime,'2 July 2010 00:02:00')

) AS a(String1,String2,Date1)

ORDER BY

CASE

WHEN @sortby = 'String1' THEN String2

WHEN @sortby = 'String2' THEN String1

end,

CASE

WHEN @sortby = 'Date1' THEN Date1

END

Thanks & Regards

Deepak.A