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, you cannot use DROP to delete the field.

The ADD clause is used in MySQL 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 is automatically added to the end of the Datasheet field.

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 location of the new field, you can use the keyword FIRST provided by MySQL (set to locate the first column) and the name of the AFTER field (set after a field).

Try the following ALTER TABLE statement, and after successful execution, use SHOW COLUMNS to see 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 ADD and MODIFY clauses, so if you want to reset the position of a datasheet field, you need to use DROP to delete the field and then use ADD to add the field and set the location.

Modify field type and name ¶

If you need to change 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), you can execute the following command:

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

Using the CHANGE clause, the syntax is very different. The CHANGE keyword is followed by the name of the field 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;

The influence of ALTER TABLE on Null values and default values ¶

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

The following example specifies that 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 the default value, MySQL will automatically set the field to NULL by default.

Modify field default values ¶

You can use ALTER to modify the default values of fields. Try the following example:

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. Try the following example, and we change the type of table testalter_tbl to MYISAM:

注意： 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 change the name of the data table, you can use the RENAME clause in the ALTER TABLE statement to do so.

Try the following example to rename the datasheet 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 on MySQL tables, which we will cover in the following sections.