1 建立表
文法:
CREATE TABLE table_name (
field1 datatype,
field2 datatype,
field3 datatype
) character set 字元集 collate 校驗規則 engine 存儲引擎;
說明:
- field 表示列名
- datatype 表示列的類型
- character set 字元集,如果沒有指定字元集,則以所在資料庫的字元集為準
- collate 校驗規則,如果沒有指定校驗規則,則以所在資料庫的校驗規則為準
2 建立表案例
create table users(
-> id int,
-> name varchar(32) comment'使用者名',
-> password char(32) comment '密碼是32位',
-> birthday date comment '生日'
-> )character set utf8 engine MYISAM;
說明:
- 不同的存儲引擎,建立表的檔案不一樣。
users 表存儲引擎是 MyISAM ,在資料目中有三個不同的檔案,分别是:
- users.frm:表結構
- users.MYD:表資料
- users.MYI:表索引
建立一個engine是innodb的資料庫,觀察存儲目錄。
3 檢視表結構
mysql> desc users;
4 修改表
在項目實際開發中,經常修改某個表的結構,比如字段名字,字段大小,字段類型,表的字元集類型,表的存儲引
擎等等。我們還有需求,添加字段,删除字段等等。這時我們就需要修改表。
ALTER TABLE tablename ADD (column datatype [DEFAULT expr][,column datatype]...);
ALTER TABLE tablename MODIfy (column datatype [DEFAULT expr][,column datatype]...);
ALTER TABLE tablename DROP (column);
案例:
- 在users表添加二條記錄
mysql> insert into users values(1, 'liyulong', '10086', '1995-10-03'),(2, 'zhangsan', '10058', '1955-12-20');
- 在users表添加一個字段,用于儲存圖檔路徑
mysql> alter table users add assets varchar(100) comment '圖檔路徑' after birthday;
- 在users中插入年齡字段
mysql> alter table users add yearold int comment '年齡' after id;
插入新字段後,對原來表中的資料沒有影響:
mysql> select * from users;
+------+---------+----------+----------+------------+---------+
| id | yearold | name | password | birthday | assents |
+------+---------+----------+----------+------------+---------+
| 1 | NULL | liyulong | 10086 | 1995-10-03 | NULL |
| 2 | NULL | zhangsan | 10058 | 1955-12-20 | NULL |
+------+---------+----------+----------+------------+---------+
- 修改name,将其長度改成60
mysql> alter table users modify name varchar(60);
Query OK, 2 rows affected (0.04 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> desc users;
+----------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| yearold | int(11) | YES | | NULL | |
| name | varchar(60) | YES | | NULL | |
| password | char(32) | YES | | NULL | |
| birthday | date | YES | | NULL | |
| assents | varchar(100) | YES | | NULL | |
+----------+--------------+------+-----+---------+-------+
6 rows in set (0.00 sec)
- 删除password列
注意:删除字段一定要小心,删除字段及其對應的列資料都沒了
mysql> alter table users drop password;
mysql> desc users;
+----------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| yearold | int(11) | YES | | NULL | |
| name | varchar(60) | YES | | NULL | |
| birthday | date | YES | | NULL | |
| assents | varchar(100) | YES | | NULL | |
+----------+--------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
- 修改表名
mysql> alter table users rename to tt;
mysql> select * from tt;
+------+---------+----------+------------+---------+
| id | yearold | name | birthday | assents |
+------+---------+----------+------------+---------+
| 1 | NULL | liyulong | 1995-10-03 | NULL |
| 2 | NULL | zhangsan | 1955-12-20 | NULL |
+------+---------+----------+------------+---------+
2 rows in set (0.00 sec)
to:可以省掉。
- 将name列修改為xingming
mysql> alter table tt change name xingming varchar(60);
mysql> desc tt;
+----------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| yearold | int(11) | YES | | NULL | |
| xingming | varchar(60) | YES | | NULL | |
| birthday | date | YES | | NULL | |
| assents | varchar(100) | YES | | NULL | |
+----------+--------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
5 删除表
文法格式:
DROP [TEMPORARY] TABLE [IF EXISTS] tbl_name [, tbl_name] ...
示例:
mysql> show tables;
+--------------------+
| Tables_in_liyulong |
+--------------------+
| tt |
| users2 |
+--------------------+
2 rows in set (0.00 sec)
mysql> drop table users2;
Query OK, 0 rows affected (0.01 sec)
mysql> show tables;
+--------------------+
| Tables_in_liyulong |
+--------------------+
| tt |
+--------------------+
1 row in set (0.00 sec)