I learned an annoying lesson about these late last week. I’d created the dataset by dragging and dropping the necessary tables from SQL Server in Visual Studio 2005. Then I added a query to one of the tables that didn’t include every one of its columns. Unfortunately, a number of the columns my query didn’t return didn’t allow nulls. I ended up modifying my query to include those columns, even though I don’t use them. I didn’t try removing the columns from the dataset, but that probably would have worked too.