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Delete a column :
The DROP COLUMN is used to delete a column from the table.
The syntax is
ALTER TABLE tbl_name DROP col_name;
The following query drops the field marks.
mysql> ALTER TABLE student DROP COLUMN marks;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc student;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| studid | int(10) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
| address | varchar(40) | YES | | NULL | |
| phone | int(10) | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
Change a column name :
When we modify a column, we have to specify the attribute of the column again. The following example renames the name field to stud_name in the student table.
mysql> ALTER TABLE student CHANGE name stud_name VARCHAR(20);
Query OK, 0 rows affected (0.05 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc student;
+-----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+-------------+------+-----+---------+-------+
| studid | int(10) | YES | | NULL | |
| stud_name | varchar(20) | YES | | NULL | |
| marks | int(10) | YES | | NULL | |
| address | varchar(40) | YES | | NULL | |
| phone | int(10) | YES | | NULL | |
+-----------+-------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
Change a column name :
If we want to change the attribute alone, we can use the same column as in the following example.
mysql> alter table student change name name varchar(40);
Query OK, 0 rows affected (0.05 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc student;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| studid | int(10) | YES | | NULL | |
| name | varchar(40) | YES | | NULL | |
| marks | int(10) | YES | | NULL | |
| address | varchar(40) | YES | | NULL | |
| phone | int(10) | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
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