1. 背景

* 視圖是數據庫中的虛擬表。包含一系列帶有名稱的行和列數據。視圖是從一個或者多個表中導出的，視圖的行為與表非常相似，用戶可以使用SELECT語句查詢數據，以及使用INSERT、UPDATE和DELETE修改記錄，視圖使用戶操作更加方便而且保障數據庫系統的安全。

* 視圖一旦定義便存儲在數據庫中，與其對應的數據並沒有像表那樣在數據庫中再存儲一份，通過視圖看到的數據只是存放在基本表中的數據。對視圖的操作與對表的操作一樣，可以對其進行查詢、修改和刪除。當對通過視圖看到的數據進行修改時，相應的基本表的數據也要發生變化，同時若基本表的數據放生變化，則這種變化也會自動地反應到視圖中。

2. 視圖作用

* 使查詢變得很清晰，視圖中存放的數據就是我們所要的數據，並且可以簡化用戶操作。

* 讓數據更安全，視圖中的數據，不存在視圖中，還是在基本表裏面，通過視圖這層關系，我們可以有效的保護我們的重要數據

* 提高表的邏輯獨立性，視圖可以屏蔽原有表結構變化帶來的影響

3. 視圖類型

* MERGE: 將引用視圖的語句的文本與視圖定義合並起來，使得視圖定義的某一部分取代語句的對應部分。

* TEMPTABLE: 將視圖的結果將被置於臨時表中，然後使用它執行語句。

* UNDEFINED：默認所使用的算法。MySQL更傾向於MERGE而不是TEMPTABLE，這是因為MERGE通常更有效。

4. 視圖類型例子

* 創建基表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)

* 對基表 users 插入數據

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

* 查看基表數據

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)

* 創建用戶id和name vusers1視圖，類型為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 視圖執行計劃

[ 未使用臨時表 ]

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)

* 創建用戶id和name vusers2視圖，類型為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 視力執行計劃

[ 未使用臨時表 ]

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)

* 創建用戶id和name vusers3視圖，類型為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 視力執行計劃

[ 使用了臨時表 ]

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. 查看視圖信息

* desc 查看視圖基本信息

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 查看視圖基本信息

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 查看視圖信息

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. 總結

以需求驅動技術，技術本身沒有優略之分，只有業務之分。

本文出自 “sea” 博客，請務必保留此出處http://lisea.blog.51cto.com/5491873/1944679

MySQL DDL操作--------視圖最佳實戰