Alter column, add default constraint

Question

I have a table and one of the columns is \"Date\" of type datetime. We decided to add a default constraint to that column

Alter table TableName
alter column         


        

Answer 1

I use the stored procedure below to update the defaults on a column.

It automatically removes any prior defaults on the column, before adding the new default.

Examples of usage:

-- Update default to be a date.
exec [dbo].[AlterDefaultForColumn] '[dbo].[TableName]','Column','getdate()';
-- Update default to be a number.
exec [dbo].[AlterDefaultForColumn] '[dbo].[TableName]','Column,'6';
-- Update default to be a string. Note extra quotes, as this is not a function.
exec [dbo].[AlterDefaultForColumn] '[dbo].[TableName]','Column','''MyString''';

Stored procedure:

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO

-- Sample function calls:
--exec [dbo].[AlterDefaultForColumn] '[dbo].[TableName]','ColumnName','getdate()';
--exec [dbol].[AlterDefaultForColumn] '[dbo].[TableName]','Column,'6';
--exec [dbo].[AlterDefaultForColumn] '[dbo].[TableName]','Column','''MyString''';
create PROCEDURE [dbo].[ColumnDefaultUpdate]
    (
        -- Table name, including schema, e.g. '[dbo].[TableName]'
        @TABLE_NAME VARCHAR(100), 
        -- Column name, e.g. 'ColumnName'.
        @COLUMN_NAME VARCHAR(100),
        -- New default, e.g. '''MyDefault''' or 'getdate()'
        -- Note that if you want to set it to a string constant, the contents
        -- must be surrounded by extra quotes, e.g. '''MyConstant''' not 'MyConstant'
        @NEW_DEFAULT VARCHAR(100)
    )
AS 
BEGIN       
    -- Trim angle brackets so things work even if they are included.
    set @COLUMN_NAME = REPLACE(@COLUMN_NAME, '[', '')
    set @COLUMN_NAME = REPLACE(@COLUMN_NAME, ']', '')

    print 'Table name: ' + @TABLE_NAME;
    print 'Column name: ' + @COLUMN_NAME;
    DECLARE @ObjectName NVARCHAR(100)
    SELECT @ObjectName = OBJECT_NAME([default_object_id]) FROM SYS.COLUMNS
    WHERE [object_id] = OBJECT_ID(@TABLE_NAME) AND [name] = @COLUMN_NAME;

    IF @ObjectName <> '' 
    begin
        print 'Removed default: ' + @ObjectName;
        --print('ALTER TABLE ' + @TABLE_NAME + ' DROP CONSTRAINT ' + @ObjectName)
        EXEC('ALTER TABLE ' + @TABLE_NAME + ' DROP CONSTRAINT ' + @ObjectName)
    end

    EXEC('ALTER TABLE ' + @TABLE_NAME + ' ADD  DEFAULT (' + @NEW_DEFAULT + ') FOR ' + @COLUMN_NAME)
    --print('ALTER TABLE ' + @TABLE_NAME + ' ADD  DEFAULT (' + @NEW_DEFAULT + ') FOR ' + @COLUMN_NAME)
    print 'Added default of: ' + @NEW_DEFAULT;
END

Errors this stored procedure eliminates

If you attempt to add a default to a column when one already exists, you will get the following error (something you will never see if using this stored proc):

-- Using the stored procedure eliminates this error:
Msg 1781, Level 16, State 1, Line 1
Column already has a DEFAULT bound to it.
Msg 1750, Level 16, State 0, Line 1
Could not create constraint. See previous errors.