當我們需要針對mysql的表名或者欄位名需要修改的時候，這個時候就需要用到mysql的alter命令。
為了方便起見，首先我們新建一張使用者表。

create table user(
`id` INT NOT NULL AUTO_INCREMENT,
`court_id` INT NOT NULL DEFAULT 1,
`user_id` INT NOT NULL DEFAULT 0,
PRIMARY KEY(`id`)
)
  ENGINE = InnoDB
  DEFAULT CHARSET = utf8;

1. 新增刪除欄位

首先如果我們想刪除user_id欄位：

mysql> alter table user drop user_id;
Query OK, 0 rows affected (0.26 sec)
Records: 0  Duplicates: 0  Warnings: 0

 
mysql> desc user;
+----------+---------+------+-----+---------+----------------+
| Field    | Type    | Null | Key | Default | Extra          |
+----------+---------+------+-----+---------+----------------+
| id       | int(11) | NO   | PRI | NULL    | auto_increment |
| court_id | int(11) | NO   |     | 1       |                |
+----------+
 
---------+------+-----+---------+----------------+
2 rows in set (0.01 sec)

注意：如果資料表中只剩餘一個欄位則無法使用DROP來刪除欄位。

我們發現user_id刪除錯誤，想添加回來：

mysql> alter table user add user_id int;
Query OK, 0 rows affected (0.25 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc user;
+----------+---------+------+-----+---------+----------------+
 

| Field    | Type    | Null | Key | Default | Extra          |
+----------+---------+------+-----+---------+----------------+
| id       | int(11) | NO   | PRI | NULL    | auto_increment |
| court_id | int(11) | NO   |     | 1       |                |
| user_id  | int(11) | YES  |     | NULL    |                |
+----------+---------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

可以看出來，此時user_id自動新增到了表的最後。

如果我們想指定新增欄位的位置，可以使用MySQL提供的關鍵字 FIRST (設定位第一列)， AFTER 欄位名（設定位於某個欄位之後）。

mysql> alter table user drop user_id;
Query OK, 0 rows affected (0.30 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table user add user_id int first;
Query OK, 0 rows affected (0.24 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc user;
+----------+---------+------+-----+---------+----------------+
| Field    | Type    | Null | Key | Default | Extra          |
+----------+---------+------+-----+---------+----------------+
| user_id  | int(11) | YES  |     | NULL    |                |
| id       | int(11) | NO   | PRI | NULL    | auto_increment |
| court_id | int(11) | NO   |     | 1       |                |
+----------+---------+------+-----+---------+----------------+
3 rows in set (0.01 sec)

此時first已經指定user_id為第一個欄位。

mysql> alter table user drop user_id;
Query OK, 0 rows affected (0.27 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table user add user_id int after id;
Query OK, 0 rows affected (0.36 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc user;
+----------+---------+------+-----+---------+----------------+
| Field    | Type    | Null | Key | Default | Extra          |
+----------+---------+------+-----+---------+----------------+
| id       | int(11) | NO   | PRI | NULL    | auto_increment |
| user_id  | int(11) | YES  |     | NULL    |                |
| court_id | int(11) | NO   |     | 1       |                |
+----------+---------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

此時可以看到，user_id位於id後面了。

2.修改欄位的名稱型別

ALTER命令中使用 MODIFY 或 CHANGE 子句能滿足以上需求。

mysql> alter table user modify court_id char(16);
Query OK, 0 rows affected (0.25 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc user;
+----------+----------+------+-----+---------+----------------+
| Field    | Type     | Null | Key | Default | Extra          |
+----------+----------+------+-----+---------+----------------+
| id       | int(11)  | NO   | PRI | NULL    | auto_increment |
| user_id  | int(11)  | YES  |     | NULL    |                |
| court_id | char(16) | YES  |     | NULL    |                |
+----------+----------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

change的語法比較奇葩。請看下面的例子

mysql> alter table user change court_id court_id int;
Query OK, 0 rows affected (0.24 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc user;
+----------+---------+------+-----+---------+----------------+
| Field    | Type    | Null | Key | Default | Extra          |
+----------+---------+------+-----+---------+----------------+
| id       | int(11) | NO   | PRI | NULL    | auto_increment |
| user_id  | int(11) | YES  |     | NULL    |                |
| court_id | int(11) | YES  |     | NULL    |                |
+----------+---------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

在 CHANGE 關鍵字之後，緊跟著的是你要修改的欄位名，必須要指定新欄位名及型別，即使欄位名不變！

3.修改欄位的預設值

為欄位新增預設值

mysql> alter table user alter court_id set default 1;
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc user;
+----------+---------+------+-----+---------+----------------+
| Field    | Type    | Null | Key | Default | Extra          |
+----------+---------+------+-----+---------+----------------+
| id       | int(11) | NO   | PRI | NULL    | auto_increment |
| user_id  | int(11) | YES  |     | NULL    |                |
| court_id | int(11) | YES  |     | 1       |                |
+----------+---------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

刪除某個欄位的預設值

mysql> alter table user alter court_id drop default;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc user;
+----------+---------+------+-----+---------+----------------+
| Field    | Type    | Null | Key | Default | Extra          |
+----------+---------+------+-----+---------+----------------+
| id       | int(11) | NO   | PRI | NULL    | auto_increment |
| user_id  | int(11) | YES  |     | NULL    |                |
| court_id | int(11) | YES  |     | NULL    |                |
+----------+---------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

4.修改表名

可以在 ALTER TABLE 語句中使用 RENAME 子句來實現修改表名的目的。

mysql> alter table user rename to user_test;
Query OK, 0 rows affected (0.05 sec)

5.修改表的引擎

可以在ALTER TABLE中設定ENGINE屬性，得到改變資料表引擎的目的。

mysql> alter table user_test ENGINE = MYISAM;
Query OK, 0 rows affected (0.18 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show table status like 'user_test'\G
*************************** 1. row ***************************
           Name: user_test
         Engine: MyISAM
        Version: 10
     Row_format: Fixed
           Rows: 0
 Avg_row_length: 0
    Data_length: 0
Max_data_length: 3659174697238527
   Index_length: 1024
      Data_free: 0
 Auto_increment: 1
    Create_time: 2018-06-05 15:36:17
    Update_time: 2018-06-05 15:36:17
     Check_time: NULL
      Collation: utf8_general_ci
       Checksum: NULL
 Create_options: 
        Comment: 
1 row in set (0.00 sec)