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Table of Contents
Add a new column (ADD COLUMN)
Modify existing columns (MODIFY COLUMN)
DROP COLUMN
Other tips
Home Database Mysql Tutorial Adding, Modifying, or Deleting Columns with ALTER TABLE in MySQL

Adding, Modifying, or Deleting Columns with ALTER TABLE in MySQL

Jul 15, 2025 am 01:14 AM
mysql

MySQL's ALTER TABLE statement is used to adjust the table structure and supports adding, modifying, and deleting columns. 1. Use ADD COLUMN to add a new column, and you can specify the location; 2. Use MODIFY COLUMN to modify the column, and you need to pay attention to data conversion and dependency objects; 3. Use DROP COLUMN to delete the column, and the operation is irreversible and the dependency will be cleared; 4. You can perform multiple operations at once, and it is recommended to check the structure through DESCRIBE or SHOW CREATE TABLE. You should verify it in the test environment before operating in the production environment.

Adding, Modifying, or Deleting Columns with ALTER TABLE in MySQL

MySQL's ALTER TABLE statement is very practical, especially when you need to adjust the table structure. For example, if you have built a user table, and later you find that there is a missing mobile number field, or a certain field type is inappropriate, you can use ALTER TABLE to add, modify or delete columns.

Adding, Modifying, or Deleting Columns with ALTER TABLE in MySQL

The following are some common operations instructions and suggestions, suitable for use when structural adjustments are encountered in daily development.

Adding, Modifying, or Deleting Columns with ALTER TABLE in MySQL

Add a new column (ADD COLUMN)

If you want to add a field to an existing table, you can use ADD COLUMN .

The syntax is roughly like this:

Adding, Modifying, or Deleting Columns with ALTER TABLE in MySQL
 ALTER TABLE Table name ADD COLUMN New column name data type constraints;

For example, suppose you have a table called users and now you want to add a phone field:

 ALTER TABLE users ADD COLUMN phone VARCHAR(20);

By default, new columns are added at the end of the table. If you want to specify a location, you can add AFTER 某列名or FIRST :

  • AFTER created_at : add the new column after created_at
  • FIRST : Put the new column in the front

It should be noted that adding columns, especially when you have default values or non-empty constraints, may lock tables or take a long time. Be careful to affect online services when the data volume is large.


Modify existing columns (MODIFY COLUMN)

If you find that a field has a wrong type, such as it was originally VARCHAR(50) , but now you want to change it VARCHAR(100) , or if you want to change the order of the fields, whether it is allowed to be empty, etc., you need to use MODIFY COLUMN .

The basic writing method is as follows:

 ALTER TABLE Table name MODIFY COLUMN Column name new data type new constraint;

For example, change phone field from VARCHAR(20) CHAR(11) and set to non-empty:

 ALTER TABLE users MODIFY COLUMN phone CHAR(11) NOT NULL;

A few points to note:

  • Modifying the field type may trigger data conversion, such as converting from a string to a number. If the processing is not done well, it will lead to data loss or errors.
  • If the field has an index or is referenced by a view or stored procedure, after modification, check whether these dependencies are still valid.
  • Just like adding columns, modifying columns may also lead to table reconstruction. Be cautious when the data volume is large.

DROP COLUMN

Sometimes some fields are no longer needed and can be deleted directly. Using DROP COLUMN :

 ALTER TABLE table name DROP COLUMN column name;

For example, delete the phone field in the users table:

 ALTER TABLE users DROP COLUMN phone;

Note before deleting:

  • This operation is irreversible. Once executed, the fields and data inside are completely deleted.
  • If there are indexes, triggers, views and other dependencies on this column, they will also be deleted together, but some versions of MySQL may report errors
  • It is recommended to confirm whether other database objects depend on this column before deleting

Other tips

  • Modify multiple columns at once?
    ALTER TABLE supports performing multiple operations at once, only need to be separated by commas:

     ALTER TABLE users 
      ADD COLUMN age INT,
      DROP COLUMN phone;
  • Check the current table structure?
    You can use DESCRIBE 表名; or SHOW CREATE TABLE 表名; to confirm the current structure to avoid misoperation.

  • Test environment verification first?
    Before executing in the production environment, it is best to run SQL in the test environment to see if there are any errors or data exceptions.


  • Basically that's it. ALTER TABLE is a very common function, but the scope of impact must be evaluated before operating in actual projects. Especially for running services, adding or re-arranged can sometimes cause performance fluctuations.

    The above is the detailed content of Adding, Modifying, or Deleting Columns with ALTER TABLE in MySQL. For more information, please follow other related articles on the PHP Chinese website!

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