Listen up my people. NOT NULL columns are your friend. It may take a few extra brain cells to populate it initially but the long term rewards in data quality are well worth it. (You'll also force all your software developer cohorts to think specifically about the values they'll be inserting into a column which is always a good thing).
But wait! I have an existing table I want to add a column to. How can I add a NOT NULL column to an existing table without dropping and reloading the table? Have no fear true believer. SQL Server can accommodate this scenario with ease. Simply, craft a little T-SQL which will add the column and the default constraint at the same time. It's as easy as pie. (Somehow I got in the habit of some sort of pop culture tie in in most of these posts. I don't really have something concrete this time but since I did say "pie": Have you seen Waitress? It's great movie with a tragic real life story involving the actress/writer/director Adrienne Shelly).
Need to add a NOT NULL column to an existing table which already has rows? Here you go:
ALTER TABLE dbo.TableA ADD NewColumnA VARCHAR(100) NOT NULL CONSTRAINT DF_TableA_NewColumnA DEFAULT 'n/a';
This code will add the new NOT NULL column NewColumnA to the table TableA. It will also add a default constraint to the column that will place the value "n/a" in the column for all the existing rows.