sql server 判断及增加列的默认值约束

IF NOT EXISTS ( SELECT name FROM sysobjects WHERE id = ( SELECT syscolumns.cdefault FROM sysobjects 
    INNER JOIN syscolumns ON sysobjects.Id=syscolumns.Id 
    WHERE sysobjects.name=N'tableName' AND syscolumns.name=N'columnName' ) 
)
BEGIN    
    ALTER TABLE tableName add constraint DF_columnName DEFAULT N'N' for columnName;  
    PRINT N'New add DF_columnName';
END
ELSE
BEGIN
    PRINT N'DEFAULT FOR columnName HAD EXISTS';
END