I'm using SQL Server 2008 and I have a table with three columns: Length, StartTime and EndTime. I want to make a CHECK constraint on this table which says that:

if Length == NULL then
  StartTime <> NULL and EndTime <> NULL
else
  StartTime == NULL and EndTime == NULL

I've begun to try things like this:

Length == NULL AND StartTime <> NULL AND EndTime <> NULL

Obviously this is not enough, but even this simple expression will not validate. I get the error:

"Error validating 'CK_Test_Length_Or_Time'. Do you want to edit the constraint?"

Any ideas on how to go about doing this?