How to modify the default value of added columns in SQL?

SQL to modify the default value of added columns cannot be modified directly. The following steps are required: Use the ALTER TABLE statement to modify the default value of newly inserted data with ALTER COLUMN. Use the UPDATE statement to update existing data and then modify the default values, but be cautious when updating large amounts of data. Consider complex situations such as data type changes, and do a good job in transaction control and backup.

How to modify the default value of added columns in SQL?

You must have encountered this situation: there is already a column in the database, but you need to modify its default value. This is not as easy as simply "changing a number", there are many pitfalls hidden in it. This article will talk about how to solve this problem gracefully, and share some of my years of experience and lessons learned from database development to help you avoid detours.

Let’s talk about the conclusion first: directly modifying the default value will not work in many cases, and may even cause your database to crash. Why? Because the database system must consider the compatibility of existing data. You directly change the default value. The column values ​​of those existing data have not changed as well! It's like you change the house number, but the letter will still be sent to the old address, it's a mess.

Therefore, we have to save the country in a curve. The most commonly used method is to modify it using the ALTER TABLE statement in conjunction with ALTER COLUMN . But this is not a one-time solution.

Core operations:

Suppose your table is called users and the column is called status , and you want to change the default value from 0 1 . You might write this:

 <code class="sql">ALTER TABLE users ALTER COLUMN status SET DEFAULT 1;</code>

It looks simple and straightforward, but it only modified the default value of the newly inserted data in the future . Their status column values ​​remain unchanged for existing data. This is usually the desired effect, but you have to know it.

Going further:

If you want to change status column value of all existing data to 1 , you have to update the data first and then change the default value:

 <code class="sql">UPDATE users SET status = 1 WHERE status IS NULL OR status  1; -- 更新所有不等于1或为空的数据ALTER TABLE users ALTER COLUMN status SET DEFAULT 1; -- 然后修改默认值</code>

This seems to solve the problem perfectly, but in practice, you have to carefully consider the amount of data. If your users table has millions or even tens of millions of data, this UPDATE statement may cause your database to be stuck for a long time, and may even cause the database to be unavailable. Therefore, be sure to execute during the low peak period, or consider batch updates.

Some pitfalls and suggestions:

• Data type change: If you want to change the data type together, such as changing INT VARCHAR , then things will be even more complicated. You need to check whether the existing data meets the new data type first, and if it does not meet, it will be processed first. This requires more meticulous planning and testing, and even data migration needs to be considered.
• Transaction control: All these operations should be placed in transactions to ensure data consistency. In case of an error in the middle, the transaction can be rolled back to avoid data corruption.
• Backup! Backup! Backup! Say important things three times. Before performing any database modification operations, be sure to make a backup. This allows you to quickly return to your previous state when something goes wrong.

Advanced Tips:

For super-large databases, you can consider using the database's own batch update tool or asynchronous update mechanism to avoid blocking the main database. This requires a deeper understanding of your database system.

In short, modifying the default value of added columns seems simple, but in actual operation, you need to be careful and fully consider various situations to avoid unnecessary trouble. Remember, good database design and operation habits can help you avoid many pitfalls. Hope these experiences help you!

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