MySQL DDL操作--------视图最佳实战

1. 背景
* 视图是数据库中的虚拟表。包含一系列带有名称的行和列数据。视图是从一个或者多个表中导出的，视图的行为与表非常相似，用户可以使用SELECT语句查询数据，以及使用INSERT、UPDATE和DELETE修改记录，视图使用户操作更加方便而且保障数据库系统的安全。
* 视图一旦定义便存储在数据库中，与其对应的数据并没有像表那样在数据库中再存储一份，通过视图看到的数据只是存放在基本表中的数据。对视图的操作与对表的操作一样，可以对其进行查询、修改和删除。当对通过视图看到的数据进行修改时，相应的基本表的数据也要发生变化，同时若基本表的数据放生变化，则这种变化也会自动地反应到视图中。

2. 视图作用
* 使查询变得很清晰，视图中存放的数据就是我们所要的数据，并且可以简化用户操作。
* 让数据更安全，视图中的数据，不存在视图中，还是在基本表里面，通过视图这层关系，我们可以有效的保护我们的重要数据
* 提高表的逻辑独立性，视图可以屏蔽原有表结构变化带来的影响




3. 视图类型

* MERGE: 将引用视图的语句的文本与视图定义合并起来，使得视图定义的某一部分取代语句的对应部分。
* TEMPTABLE: 将视图的结果将被置于临时表中，然后使用它执行语句。
* UNDEFINED：默认所使用的算法。MySQL更倾向于MERGE而不是TEMPTABLE，这是因为MERGE通常更有效。

4. 视图类型例子
* 创建基表users

mysql> CREATE TABLE users(
-> id BIGINT PRIMARY KEY NOT NULL AUTO_INCREMENT,
-> name VARCHAR(64) NOT NULL,
-> sex ENUM('M', 'F') NOT NULL,
-> age INT NOT NULL
-> )ENGINE=INNODB CHARSET=utf8mb4;
Query OK, 0 rows affected (0.15 sec)

* 对基表 users 插入数据

mysql> INSERT INTO users VALUES(NULL, 'tom', 'M', 23), (NULL, 'jak', 'F', 32), (NULL, 'jus', 'M', 35);
Query OK, 3 rows affected (0.04 sec)
Records: 3  Duplicates: 0  Warnings: 0

* 查看基表数据

mysql> SELECT * FROM users;
+----+------+-----+-----+
| id | name | sex | age |
+----+------+-----+-----+
|  1 | tom  | M   |  23 |
|  2 | jak  | F   |  32 |
|  3 | jus  | M   |  35 |
+----+------+-----+-----+
3 rows in set (0.00 sec)

* 创建用户id和name vusers1视图，类型为UNDEFINED

mysql> CREATE ALGORITHM=UNDEFINED VIEW vusers1 AS SELECT id, name FROM users;
Query OK, 0 rows affected (0.01 sec)

* 分析查看 vusers1 视图执行计划
[ 未使用临时表 ]

mysql> EXPLAIN SELECT * FROM vusers1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | users | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

* 创建用户id和name vusers2视图，类型为merge

mysql> CREATE ALGORITHM=MERGE VIEW vuser2 AS SELECT id, name FROM users;
Query OK, 0 rows affected (0.03 sec)

* 分析查看 vusers2 视力执行计划
[ 未使用临时表 ]

mysql> EXPLAIN SELECT * FROM vuser2;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | users | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)

* 创建用户id和name vusers3视图，类型为TEMPTABLE

mysql> CREATE ALGORITHM=TEMPTABLE VIEW vuser3 AS SELECT id, name FROM users;
Query OK, 0 rows affected (0.19 sec)

* 分析查看 vusers3 视力执行计划
[ 使用了临时表 ]

mysql> EXPLAIN SELECT * FROM vuser3;
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table      | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | PRIMARY     | <derived2> | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | NULL  |
|  2 | DERIVED     | users      | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | NULL  |
+----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)

5. 查看视图信息
* desc 查看视图基本信息

mysql> desc vusers1;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | bigint(20)  | NO   |     | 0       |       |
| name  | varchar(64) | NO   |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

* show table status 查看视图基本信息

mysql> show table status like 'vusers1';
+--------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+-------------+-------------+------------+-----------+----------+----------------+---------+
| Name   | Engine | Version | Row_format | Rows | Avg_row_length | Data_length | Max_data_length | Index_length | Data_free | Auto_increment | Create_time | Update_time | Check_time | Collation | Checksum | Create_options | Comment |
+--------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+-------------+-------------+------------+-----------+----------+----------------+---------+
| vuser1 | NULL   |    NULL | NULL       | NULL |           NULL |        NULL |            NULL |         NULL |      NULL |           NULL | NULL        | NULL        | NULL       | NULL      |     NULL | NULL           | VIEW    |
+--------+--------+---------+------------+------+----------------+-------------+-----------------+--------------+-----------+----------------+-------------+-------------+------------+-----------+----------+----------------+---------+
1 row in set (0.00 sec)

* show create view 查看视图信息

mysql> show create view vuser1;
+--------+---------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+
| View   | Create View                                                                                                                                                   | character_set_client | collation_connection |
+--------+---------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+
| vuser1 | CREATE ALGORITHM=UNDEFINED DEFINER=`root`@`localhost` SQL SECURITY DEFINER VIEW `vuser1` AS select `users`.`id` AS `id`,`users`.`name` AS `name` from `users` | utf8                 | utf8_general_ci      |
+--------+---------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+
1 row in set (0.00 sec)

6. 总结以需求驱动技术，技术本身没有优略之分，只有业务之分。