mysql 如果字段 存在 删除 mysql表删除字段

1.增加一个字段

alter table user add COLUMN new1 VARCHAR(20) DEFAULT NULL; //增加一个字段，默认为空
alter table user add COLUMN new2 VARCHAR(20) NOT NULL;  //增加一个字段，默认不能为空

 
2.删除一个字段

alter table user DROP COLUMN new2; 　 //删除一个字段

 
3.修改一个字段

alter table user MODIFY new1 VARCHAR(10); 　//修改一个字段的类型
 
alter table user CHANGE new1 new4 int;　 //修改一个字段的名称，此时一定要重新指定该字段的类型

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1增加两个字段：

1. 
   

1. mysql> create table id_name(id int,name varchar(20));  
2. Query OK, 0 rows affected (0.13 sec)  
3.   
4.   
5. mysql> alter table id_name add age int,add address varchar(11);  
6. Query OK, 0 rows affected (0.13 sec)  
7. Records: 0  Duplicates: 0  Warnings: 0  
8.   
9. mysql> desc id_name；  
10.   
11. +---------+-------------+------+-----+---------+-------+  
12. | Field   | Type        | Null | Key | Default | Extra |  
13. +---------+-------------+------+-----+---------+-------+  
14. | id      | int(11)     | YES  |     | NULL    |       |  
15. | name    | varchar(20) | YES  |     | NULL    |       |  
16. | age     | int(11)     | YES  |     | NULL    |       |  
17. | address | varchar(11) | YES  |     | NULL    |       |  
18. +---------+-------------+------+-----+---------+-------+  
19. 4 rows in set (0.00 sec)  
20.   
21. 2.删除两个字段  
22. mysql> alter table id_name drop column age,drop column address;  
23. Query OK, 0 rows affected (0.14 sec)  
24. Records: 0  Duplicates: 0  Warnings: 0  
25.   
26. mysql> desc id_name;  
27. +-------+-------------+------+-----+---------+-------+  
28. | Field | Type        | Null | Key | Default | Extra |  
29. +-------+-------------+------+-----+---------+-------+  
30. | id    | int(11)     | YES  |     | NULL    |       |  
31. | name  | varchar(20) | YES  |     | NULL    |       |  
32. +-------+-------------+------+-----+---------+-------+  
33. 2 rows in set (0.00 sec)  
34.   
35. 3.插入  
36. mysql> insert into id_name values (1,'qustdjx');  
37. Query OK, 1 row affected (0.00 sec)  
38. 4.查询看一下  
39. mysql> alter table id_name add age int,add address varchar(11);  
40. Query OK, 1 row affected (0.07 sec)  
41. Records: 1  Duplicates: 0  Warnings: 0  
42.   
43. mysql> select * from id_name;  
44. +------+---------+------+---------+  
45. | id   | name    | age  | address |  
46. +------+---------+------+---------+  
47. |    1 | qustdjx | NULL | NULL    |  
48. +------+---------+------+---------+  
49. 1 row in set (0.00 sec)  
50. 5.新增字段并插入  
51. mysql> insert into id_name values(2,'qust',14,'山东');  
52. Query OK, 1 row affected (0.00 sec)  
53.   
54. mysql> select * from id_name;  
55. +------+---------+------+---------+  
56. | id   | name    | age  | address |  
57. +------+---------+------+---------+  
58. |    1 | qustdjx | NULL | NULL    |  
59. |    2 | qust    |   14 | 山东    |  
60. +------+---------+------+---------+  
61. 2 rows in set (0.00 sec)  
62.