MySQL学习笔记(三)

8. 创建和操纵表

8.1 创建表

mysql> create table test(
    -> t_id int(10) not null auto_increment,
    -> t_name char(10) null default "",
    -> t_text text null,
    -> t_time datetime not null,
    -> primary key(t_id),
    -> fulltext(t_text))engine=MyISAM;
Query OK, 0 rows affected (0.22 sec)

主键可以在创建的时候用primary key()来标识，如果主键使用单个列，那它的值必须唯一。创建由多个列组成的主键，应该用逗号进行分割。例如primary key(t_id,t_time)。允许NULL值的列也允许在插入行时不给出该列的值。

每个表只允许一个auto_increment，而且它必须被索引(通常使它成为主键)。

8.2 指定默认值

利用default关键词来指定默认值（MySQL只支持常量默认值）

8.3 引擎类型

与其他DBMS一样，MySQL有一个具体管理和处理数据的内部引擎。当你使用create table语句时，该引擎具体创建表，而在你使用select语句或者使用其它数据库处理时，该引擎在内部处理你的请求。多数时候，此引擎都隐藏在DBMS内，不需要过多关注它。

以下是几个需要知道的引擎：

1. InnoDB 是一个可靠的事务处理引擎，它不支持全文搜索。

2. Memory在功能上等同于MyISAM，但由于数据存储在内存（不是磁盘）中，速度很快（特别适合临时表）。

3. MyISAM是一个性能极高的引擎，它支持全文本搜索，但不支持事务处理。

8.4 更新表

利用Alter table+表名更改表结构，区别于利用Update +表名更新表内的数据

8.4.1 给表添加一个列

mysql> alter table test add column t_extra char(24) null default "";
Query OK, 0 rows affected (0.19 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> describe test;
+---------+----------+------+-----+---------+----------------+
| Field   | Type     | Null | Key | Default | Extra          |
+---------+----------+------+-----+---------+----------------+
| t_id    | int(10)  | NO   | PRI | NULL    | auto_increment |
| t_name  | char(10) | YES  |     |         |                |
| t_text  | text     | YES  | MUL | NULL    |                |
| t_time  | datetime | NO   |     | NULL    |                |
| t_extra | char(24) | YES  |     |         |                |
+---------+----------+------+-----+---------+----------------+
5 rows in set (0.06 sec)

8.4.2 删除表的一个列

mysql> alter table test drop column
    -> t_extra;
Query OK, 0 rows affected (0.07 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> describe test;
+--------+----------+------+-----+---------+----------------+
| Field  | Type     | Null | Key | Default | Extra          |
+--------+----------+------+-----+---------+----------------+
| t_id   | int(10)  | NO   | PRI | NULL    | auto_increment |
| t_name | char(10) | YES  |     |         |                |
| t_text | text     | YES  | MUL | NULL    |                |
| t_time | datetime | NO   |     | NULL    |                |
+--------+----------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

8.4.3 Alter table最常见的用途就是定义外键

mysql> show tables;
+----------------+
| Tables_in_data |
+----------------+
| ab             |
| member         |
| newname        |
| test           |
+----------------+
4 rows in set (0.00 sec)
mysql> describe member;
+------------+-------------+------+-----+---------+----------------+
| Field      | Type        | Null | Key | Default | Extra          |
+------------+-------------+------+-----+---------+----------------+
| m_id       | int(11)     | NO   | PRI | NULL    | auto_increment |
| m_FN       | varchar(20) | YES  |     | NULL    |                |
| m_LN       | varchar(20) | YES  |     | NULL    |                |
| m_birth    | datetime    | YES  |     | NULL    |                |
| m_info     | varchar(20) | YES  |     | NULL    |                |
| extra_info | char(20)    | YES  |     | cat     |                |
+------------+-------------+------+-----+---------+----------------+
6 rows in set (0.41 sec)
mysql> alter table test
    -> add constraint fk_test_member
    -> foreign key(t_id) references member(m_id);
Query OK, 0 rows affected (0.10 sec)
Records: 0  Duplicates: 0  Warnings: 0

8.4 给表插入数据

8.4.1 插入单组数据

mysql> insert into test(t_id,t_name,t_text,t_time)
    -> values(
    -> 2,"Tom","This is a cat.",now());
Query OK, 1 row affected (0.09 sec)

mysql> select * from test;
+------+--------+----------------+---------------------+
| t_id | t_name | t_text         | t_time              |
+------+--------+----------------+---------------------+
|    2 | Tom    | This is a cat. | 2019-04-25 15:33:02 |
+------+--------+----------------+---------------------+
1 row in set (0.00 sec)

可以直接指定列名，也可以直接插入数据，不指定列名，但是此时一定要根据表中的列的顺序进行插入数据。

如果不指定例中的t_id，则因为它有auto_increment属性，则它会自增来赋值。如果某个列设置not null属性并且没有默认值，则必须给予数据，不然将会出错。

8.4.2 插入多组数据

mysql> select *from test;
+------+--------+-----------------+---------------------+
| t_id | t_name | t_text          | t_time              |
+------+--------+-----------------+---------------------+
|    4 | Jerry  | This is a mouse | 2019-04-25 15:40:55 |
+------+--------+-----------------+---------------------+
1 row in set (0.00 sec)

mysql> insert into test(t_name,t_text,t_time)
    -> values(
    -> "Tom","This is a cat",now()),
    -> ("Helen","She is a student in middle school.",now());
Query OK, 2 rows affected (0.06 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from test;
+------+--------+------------------------------------+---------------------+
| t_id | t_name | t_text                             | t_time              |
+------+--------+------------------------------------+---------------------+
|    5 | Tom    | This is a cat                      | 2019-04-25 16:13:39 |
|    4 | Jerry  | This is a mouse                    | 2019-04-25 15:40:55 |
|    6 | Helen  | She is a student in middle school. | 2019-04-25 16:13:39 |
+------+--------+------------------------------------+---------------------+
3 rows in set (0.00 sec)

8.4.3 插入检索出来的数据

mysql> select * from member;
+------+-------+--------+---------------------+---------+------------+
| m_id | m_FN  | m_LN   | m_birth             | m_info  | extra_info |
+------+-------+--------+---------------------+---------+------------+
|    1 | Halen | Park   | 1970-06-29 00:00:00 | GoodMan | cat        |
|    3 | Bob   | Garden | 2019-04-25 10:22:49 | Badman  | cat        |
+------+-------+--------+---------------------+---------+------------+
2 rows in set (0.00 sec)
mysql> insert into test(t_id,t_name,t_time)
    -> select m_id,m_FN,now() from member;
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from test;
+------+--------+------------------------------------+---------------------+
| t_id | t_name | t_text                             | t_time              |
+------+--------+------------------------------------+---------------------+
|    5 | Tom    | This is a cat                      | 2019-04-25 16:13:39 |
|    4 | Jerry  | This is a mouse                    | 2019-04-25 15:40:55 |
|    6 | Helen  | She is a student in middle school. | 2019-04-25 16:13:39 |
|    1 | Halen  | NULL                               | 2019-04-25 16:17:21 |
|    3 | Bob    | NULL                               | 2019-04-25 16:17:21 |
+------+--------+------------------------------------+---------------------+
5 rows in set (0.00 sec)

8.5 删除表

mysql> drop table ab;
Query OK, 0 rows affected (0.01 sec)

mysql> describe ab;
ERROR 1146 (42S02): Table 'data.ab' doesn't exist

8.6 重命名表名

mysql> alter table test rename to tt;
Query OK, 0 rows affected (0.03 sec)

mysql> show tables;
+----------------+
| Tables_in_data |
+----------------+
| member         |
| newname        |
| tt             |
+----------------+
3 rows in set (0.00 sec)

mysql> rename table tt to test;
Query OK, 0 rows affected (0.01 sec)

mysql> show tables;
+----------------+
| Tables_in_data |
+----------------+
| member         |
| newname        |
| test           |
+----------------+
3 rows in set (0.00 sec)

9. 更新和删除数据

更新和删除数据一定要配合where条件语句来进行，不然操作对象将是整个行记录。

9.1 更新数据

mysql> select * from test;
+------+--------+----------------+---------------------+
| t_id | t_name | t_text         | t_time              |
+------+--------+----------------+---------------------+
|    2 | Tom    | This is a cat. | 2019-04-25 15:33:02 |
+------+--------+----------------+---------------------+
1 row in set (0.00 sec)

mysql> update test
    -> set t_id=3
    -> where t_id=2;
Query OK, 1 row affected (0.08 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from test;
+------+--------+----------------+---------------------+
| t_id | t_name | t_text         | t_time              |
+------+--------+----------------+---------------------+
|    3 | Tom    | This is a cat. | 2019-04-25 15:33:02 |
+------+--------+----------------+---------------------+
1 row in set (0.00 sec)

如果不指定where条件，则set语句将会应用到所有的行

mysql> insert into test(t_name,t_text,t_time)
    -> values("Jerry","This is a mouse",now());
Query OK, 1 row affected (0.00 sec)

mysql> select * from test;
+------+--------+-----------------+---------------------+
| t_id | t_name | t_text          | t_time              |
+------+--------+-----------------+---------------------+
|    3 | Tom    | This is a cat.  | 2019-04-25 15:33:02 |
|    4 | Jerry  | This is a mouse | 2019-04-25 15:39:55 |
+------+--------+-----------------+---------------------+
2 rows in set (0.00 sec)

mysql> update test
    -> set t_time=now();
Query OK, 2 rows affected (0.00 sec)
Rows matched: 2  Changed: 2  Warnings: 0

mysql> select * from test;
+------+--------+-----------------+---------------------+
| t_id | t_name | t_text          | t_time              |
+------+--------+-----------------+---------------------+
|    3 | Tom    | This is a cat.  | 2019-04-25 15:40:55 |
|    4 | Jerry  | This is a mouse | 2019-04-25 15:40:55 |
+------+--------+-----------------+---------------------+
2 rows in set (0.00 sec)

9.2 删除数据

mysql> delete from test
    -> where t_id=3;
Query OK, 1 row affected (0.00 sec)
mysql> select *from test;
+------+--------+-----------------+---------------------+
| t_id | t_name | t_text          | t_time              |
+------+--------+-----------------+---------------------+
|    4 | Jerry  | This is a mouse | 2019-04-25 15:40:55 |
+------+--------+-----------------+---------------------+
1 row in set (0.00 sec)