如何在不丢失数据的情况下改变MySQL表？

Question

In my application, I make some changes and upload them to a testing server. Because I have no access to the server database I run ALTER commands to make changes on it.

Using a method I ran the following command on server:

ALTER TABLE `blahblahtable` ADD COLUMN `newcolumn` INT(12) NOT NULL

After that, I found that the all the data of the table has been removed. Now the table is blank.

So I need to alter the table without removing his data. Is there any way to do that?

Answer 1

Your question is quite obvious. You're adding a new column to the table, and setting it to NOT NULL .
To make things clearer, I will explain the reaction of the server when you run the command:

1. You add a new column, so every row of the table has to set a value for that column.

2. As you don't declare any default value , all the rows set null for this new column.

3. The server notices that the rows of the table have a null value on a column that doesn't allow null s. This is illegal.

4. To solve the conflict, the invalid rows are deleted .

There are some good fixes for this issue:

• Set a default value (recommended) for the column you're creating.

• Create the column without the NOT NULL , set the appropiate values, and then make the column NOT NULL .

Answer 2

您可以创建临时表，传递要更改的表中的所有信息，然后将信息返回到更改的表。