If you look at example 2 of my article you will notice that the two times I use the CASE function with a datetime field I cast the datetime field as varchar. All datetime fields must be cast as varchar (or char) when you mix datatypes in your CASE function.

So for the code you posted in the ORDER BY you should replace a.date_completed with CAST(a.date_completed as varchar(20)).

This will make your code not fail with the error you are getting, however when sorting by the datetime column converted to varchar you still won't get the sorting exactly correct so you'll probably need to manipulate the datetime column to put the year first, then the month, and finally the day so that it will sort correctly. Let me test this with example 2 and then I'll post the sample code here.

Robert Marda