There are lots of ways to learn new things, or be reminded of old things. One of my favorites is the QotD on sql server central. Recently there was an interesting question on defaults. It pointed out that there is a difference between adding a column with a default that allows NULLs and one that does not.

-- Setup
CREATE TABLE DefaultTest (Id INT NOT NULL IDENTITY(1,1))
GO

INSERT INTO DefaultTest DEFAULT VALUES
INSERT INTO DefaultTest DEFAULT VALUES
INSERT INTO DefaultTest DEFAULT VALUES
GO

-- Test adding one NULL column and one NOT NULL column
ALTER TABLE DefaultTest ADD 
	Nullable char(1) NULL 
		CONSTRAINT df_DefaultTest_Nullable DEFAULT 'x',
	NotNull char(1) NOT NULL
		CONSTRAINT df_DefaultTest_NotNull DEFAULT 'x'
GO

With the results:

Note that the nullable column is all NULLs and the other was filled in with x’s. This makes sense when you think about it. If you add a column with a default that allows NULLs it can just have NULL in any existing rows. However when you add a column that doesn’t allow NULLs then you have to have a value to put in it. In fact that brings up the point that you can’t add a NOT NULL column without a default if there are any rows in the table.

This

-- Trying to add a NOT NULL column without a default
ALTER TABLE DefaultTest ADD 
	NotNull2 char(1) NOT NULL 
GO

Returns an error of

Msg 4901, Level 16, State 1, Line 1
ALTER TABLE only allows columns to be added that can contain 
nulls, or have a DEFAULT definition specified, or the column 
being added is an identity or timestamp column, or alternatively 
if none of the previous conditions are satisfied the table must 
be empty to allow addition of this column. Column 'NotNull2' 
cannot be added to non-empty table 'DefaultTest' because it does 
not satisfy these conditions.

This is one of those simple things that can bite you in the ____ occasionally. It certainly catches me every now and again.

Edit

I noticed an interesting set of keywords on the ALTER TABLE command while looking at an answer on dba.stackexchange.

ALTER TABLE DefaultTest ADD 
	Nullable char(1) NULL 
		CONSTRAINT df_DefaultTest_Nullable DEFAULT 'x'
		WITH VALUES; 

The WITH VALUES keywords cause the column to be filled in with the default values even though the column is NULLable. FYI there is no error if you include WITH VALUES and the column is NOT NULL but it doesn’t change the behavior either.

Got to love learning new stuff!