为什么在已创建的表中添加列被视为不良做法？

I was looking at the ALTER procedure to add a column to an already-created table and a lot of the answers specified that adding columns dynamically wasn't considered good practice.
I am trying to create an extra column to monitor the attendance on a particular day so in relation to that aim, is altering the table still bad practice, because I can't really see a way around it?

Database should be created in the very begining of project and it should stay as it is. When you create your query, for example INSERT and you don't use named colummns it can crush your system.

For EXAMPLE

INSERT INTO `table` VALUES('a','b','c');

instead of

INSERT INTO `table`(column1, column2, column3,...)
VALUES ('a', 'b', 'c',...)

There are a lot of lines of code than can crash. Mostly those that are based on columns numbers instead of their names. Changing DB can create a lot of problems and if the system is pretty big they can be hard to be found.

It's better practice to create relations within tables than creating a lot of columns.

Morever you should read about Database normalization which "is the process of organizing the fields and tables of a relational database to minimize redundancy and dependency."

It's not something that should be done willy-nilly and it requires planning but if you have changing requirements or simply forgot something, by all means: add the column.

Never adding a column is simply no good advice and not realistic. You should be able to

  • Adapt to changing requirements
    Requirements only stop changing when the application is discontinued or the bussiness is bankrupt
  • Keep your database normalised
    Instead of adding a column you could decide to add an entire table referencing your existing table avoid the alledged bad practice. Please don't.
  • Fix mistakes in the specification/implementation
    we're all human

but keep in mind that adding a column

  • requires testing
  • can easily break existing applications
  • requires testing
  • requires planning
  • and requires testing

There's too ways to understand your question,

  1. you're talking about adding a column to an already-existing table, but you're not exactly doing it dynamically, on the fly from your application, but rather once when you update your application code. during deployment. This is ok as long as you keep your application and db structure in sync.
  2. you're talking about adding a column from your app, runtime, whenever some condition happens or data is in a state that would require adding a table. This is rarely a good thing to do. It might be ok in a CMS in some cases where you're creating content types that are not expected to change and are added rarely. However, mostly it's an indication of bad design and will cause both performance and complexity issues.

It depends when do you want to make these changes.

If the changes happen during because of extended features of a system, they are IMHO ok.

But if the changes are done continuously while using the application, that is bad design.

In general, you make a distinction between

  1. Data Description Language (DDL) where you make changes in the layout. These are mainly CREATE TABLE, ALTER TABLE and alike. These changes should not be done during the normal run of the application, only during setup.

  2. Data Manipulation and Query Language (DML) where you operate on the data in a DB. This is what an aplication normally does: INSERT, DELETE, SELECT.