Alter column default value

There is no direct way to change default value of a column in SQL server, but the following parameterized script will do the work:

DECLARE @table NVARCHAR(100);
DECLARE @column NVARCHAR(100);
DECLARE @newDefault NVARCHAR(100);
SET @table = N'TableName';
SET @column = N'ColumnName';
SET @newDefault = N'0';

IF EXISTS (
    SELECT   name
    FROM     sys.default_constraints
    WHERE    parent_object_id = OBJECT_ID(@table)
    AND      parent_column_id = COLUMNPROPERTY(OBJECT_ID(@table), @column, 'ColumnId')
)
BEGIN
    DECLARE @constraintName AS NVARCHAR(200);
    DECLARE @constraintQuery AS NVARCHAR(2000);

    SELECT  @constraintName = name
    FROM    sys.default_constraints
    WHERE   parent_object_id = OBJECT_ID(@table)
    AND parent_column_id = COLUMNPROPERTY(OBJECT_ID(@table), @column, 'ColumnId');

    SET @constraintQuery = N'ALTER TABLE ' + @table + N' DROP CONSTRAINT '
        + @constraintName + N'; ALTER TABLE ' + @table + N' ADD CONSTRAINT '
        + @constraintName + N' DEFAULT ' + @newDefault + N' FOR ' + @column;

    EXECUTE sp_executesql @constraintQuery;
END;

Just fill the parameters and execute. The script removes existing constraint and creates a new one with designated default value.

For altering an existing column you need to:

ALTER TABLE MyTable ALTER COLUMN CreateDate DATETIME NOT NULL;
ALTER TABLE MyTable ADD CONSTRAINT DF_Constraint DEFAULT GetDate() FOR CreateDate;

I think issue here is with the confusion between Create Table and Alter Table commands. If we look at Create table then we can add a default value and default constraint at same time as:

<column_definition> ::= 
column_name <data_type>
    [ FILESTREAM ]
    [ COLLATE collation_name ] 
    [ SPARSE ]
    [ NULL | NOT NULL ]
    [ 
        [ CONSTRAINT constraint_name ] DEFAULT constant_expression ] 
      | [ IDENTITY [ ( seed,increment ) ] [ NOT FOR REPLICATION ] 
    ]
    [ ROWGUIDCOL ]
    [ <column_constraint> [ ...n ] ] 
    [ <column_index> ]
 ex: 
CREATE TABLE dbo.Employee 
(
     CreateDate datetime NOT NULL 
     CONSTRAINT DF_Constraint DEFAULT (getdate())
) 
ON PRIMARY;

you can check for complete definition here: http://msdn.microsoft.com/en-IN/library/ms174979.aspx

but if we look at the Alter Table definition then with ALTER TABLE ALTER COLUMN you cannot add CONSTRAINT the options available for ADD are:

 | ADD 
    { 
        <column_definition>
      | <computed_column_definition>
      | <table_constraint> 
      | <column_set_definition> 
    } [ ,...n ]

Check here: http://msdn.microsoft.com/en-in/library/ms190273.aspx

So you will have to write two different statements one for Altering column as:

ALTER TABLE MyTable ALTER COLUMN CreateDate DATETIME NOT NULL;

and another for altering table and add a default constraint

ALTER TABLE MyTable ADD CONSTRAINT DF_Constraint DEFAULT GetDate() FOR CreateDate;

Hope this helps!!!