MySQL ALTER command

When we need to modify the data table name or modify the data table fields, we need to use the MySQL ALTER command.

Before starting the tutorial in this chapter, let us create a table named: testalter_tbl.


root@host# mysql -u root -p password;
Enter password:*******
mysql> use RUNOOB;
Database changed
mysql> create table testalter_tbl
    -> (
    -> i INT,
    -> c CHAR(1)
    -> );
Query OK, 0 rows affected (0.05 sec)
mysql> SHOW COLUMNS FROM testalter_tbl;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| i     | int(11) | YES  |     | NULL    |       |
| c     | char(1) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

delete, add or modify table fields

The following command uses the ALTER command and the DROP clause to delete the i field of the table created above:


mysql> ALTER TABLE testalter_tbl  DROP i;

If there is only one field left in the data table, DROP cannot be used to delete the field.

MySQL uses the ADD clause to add columns to the data table. The following example adds the i field to the table testalter_tbl and defines the data type:


mysql> ALTER TABLE testalter_tbl ADD i INT;

After executing the above command, the i field will be automatically added to the end of the data table fields.


mysql> SHOW COLUMNS FROM testalter_tbl;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| c     | char(1) | YES  |     | NULL    |       |
| i     | int(11) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

If you need to specify the position of the new field, you can use the keyword FIRST (set in the first column) and AFTER field name (set after a certain field) provided by MySQL.

Try the following ALTER TABLE statement, after successful execution, use SHOW COLUMNS to view the changes in the table structure:


ALTER TABLE testalter_tbl DROP i;
ALTER TABLE testalter_tbl ADD i INT FIRST;
ALTER TABLE testalter_tbl DROP i;
ALTER TABLE testalter_tbl ADD i INT AFTER c;

The FIRST and AFTER keywords can be used in the ADD and MODIFY clauses, so if you want to reset the position of the data table fields, you need to use DROP to delete the field first and then use ADD to add the field and set the position.

Modify field type and name

If you need to modify the field type and name, you can use the MODIFY or CHANGE clause in the ALTER command.

For example, to change the type of field c from CHAR(1) to CHAR(10), execute the following command:


mysql> ALTER TABLE testalter_tbl MODIFY c CHAR(10);

With the CHANGE clause, the syntax is quite different. After the CHANGE keyword, follow the field name you want to modify, and then specify the new field name and type. Try the following example:


mysql> ALTER TABLE testalter_tbl CHANGE i j BIGINT;


mysql> ALTER TABLE testalter_tbl CHANGE j j INT;

Effects of ALTER TABLE on Null and Default Values

When you modify a field, you can specify whether to include a value or whether to set a default value.

In the following example, the specified field j is NOT NULL and the default value is 100.


mysql> ALTER TABLE testalter_tbl 
    -> MODIFY j BIGINT NOT NULL DEFAULT 100;

If you do not set a default value, MySQL will automatically set the field to NULL by default.

Modify field default value

You can use ALTER to modify the default value of the field, try the following examples:


mysql> ALTER TABLE testalter_tbl ALTER i SET DEFAULT 1000;
mysql> SHOW COLUMNS FROM testalter_tbl;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| c     | char(1) | YES  |     | NULL    |       |
| i     | int(11) | YES  |     | 1000    |       |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

You can also use the ALTER command and the DROP clause to delete the default value of the field, as shown in the following example:


mysql> ALTER TABLE testalter_tbl ALTER i DROP DEFAULT;
mysql> SHOW COLUMNS FROM testalter_tbl;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| c     | char(1) | YES  |     | NULL    |       |
| i     | int(11) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)
Changing a Table Type:

To modify the data table type, you can use the ALTER command and the TYPE clause to complete. Try the following example, we will modify the type of table testalter_tbl to MYISAM:

**Note:** You can use the SHOW TABLE STATUS statement to view the data table type.


mysql> ALTER TABLE testalter_tbl ENGINE = MYISAM;
mysql>  SHOW TABLE STATUS LIKE 'testalter_tbl'\G
*************************** 1. row ****************
           Name: testalter_tbl
           Type: MyISAM
     Row_format: Fixed
           Rows: 0
 Avg_row_length: 0
    Data_length: 0
Max_data_length: 25769803775
   Index_length: 1024
      Data_free: 0
 Auto_increment: NULL
    Create_time: 2007-06-03 08:04:36
    Update_time: 2007-06-03 08:04:36
     Check_time: NULL
 Create_options:
        Comment:
1 row in set (0.00 sec)

modify table name

If you need to modify the name of the data table, you can use the RENAME clause in the ALTER TABLE statement to achieve.

Try the following example to rename the data table testalter_tbl to alter_tbl:


mysql> ALTER TABLE testalter_tbl RENAME TO alter_tbl;

The ALTER command can also be used to create and delete indexes of MySQL data tables, which we will introduce in the next chapter.

Happy file MySQL ALTER command