Assume we have a user table, and the user structure is as follows:
mysql> desc user;
Modify table field type modify
Category
Basic syntax
##Example
alter table user modify username varchar(20);
Example description
Change the type of username in the user table to varchar(20)
Let’s execute it and see the result:
mysql> alter table user modify username varchar(20);
Query OK, 0 rows affected (0.48 sec)
Records: 0 Duplicates: 0 Warnings: 0
+----------+---------------+------+-----+--------- +-------+
| Field | Type | Null | Key | Default | Extra |
Category
Detailed explanation
##Basic syntax
alter table table name add column field name type;
Example
alter table user add column age int(3);
Example description
Add a field as age, type is integer, length is 3
mysql> alter table emp add column age int(3);
mysql> desc user;
Control the order of fields when adding fields
Category
Basic syntax
Example
Example description
ALTER TABLE user ADD email VARCHAR(60) AFTER createip;
mysql> desc user;
删除表字段
Category
##Basic syntax
ALTER TABLE table name ADD Field name Field type;
Example
ALTER TABLE user ADD id INT(10) FIRST;
Example description
In the user table, add a field as id at the beginning, type is int, length is 10
类别
详细解示
基本语法
alter table 表名 drop column 字段名;
示例
alter table user drop column age;
示例说明
在user表中删除字段age
mysql> alter table user drop column age;
mysql> desc user;
Table field renameDetailed example:
mysql> alter table user change email em varchar(60);
mysql> desc user;
mysql> alter table user modify em varchar(60) first;
Query OK, 0 rows affected (0.41 sec)
mysql> desc user;
+----------+---------------+------+-----+--------- +-------+
Modify table name
Category
Basic syntax
Example
Example description
Modify the order of table fieldsIn the previous field addition and modification statements (add/change/modify), you can add one at the end Optional first|after.We have already learned how to adjust the order when adding table fields. Let's now take a look at how another change or modify can adjust the order.Let’s do a small experiment with first.Use modify to adjust the order
Category
Detailed explanation
Basic syntax
alter table old table name rename new table name;
Example
alter table user rename new_user;
Example description
Change the user table name to new_user
##mysql> alter table user rename new_user;
Query OK, 0 rows affected (0.35 sec)
mysql> desc new_user;
+----------+---------------+------+-----+--------- +-------+
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