为mysql中的视图创建注释

Question

我看到视图有一个注释字段，就像常规表一样，但默认情况下填充有“VIEW”值。

[TABLE_CATALOG] => 
[TABLE_SCHEMA] => xxx
[TABLE_NAME] => view__xxxx
[TABLE_TYPE] => VIEW
[ENGINE] => 
[VERSION] => 
[ROW_FORMAT] => 
[TABLE_ROWS] => 
[AVG_ROW_LENGTH] => 
[DATA_LENGTH] => 
[MAX_DATA_LENGTH] => 
[INDEX_LENGTH] => 
[DATA_FREE] => 
[AUTO_INCREMENT] => 
[CREATE_TIME] => 
[UPDATE_TIME] => 
[CHECK_TIME] => 
[TABLE_COLLATION] => 
[CHECKSUM] => 
[CREATE_OPTIONS] => 
[TABLE_COMMENT] => VIEW

当我尝试创建带有注释的视图时，出现错误。

CREATE OR REPLACE VIEW view__x AS
SELECT 
 * 
FROM `some_table`  
COMMENT = 'some comment'

有没有办法修改评论字段，或者该字段在内部用于其他用途并且应该保持原样？

我已向 mysql 添加了功能请求。

Answer 1

根据创建视图语法，当前无法为视图添加注释：

https://dev.mysql.com/doc/refman/8.1/en/create-view.html

此功能已被多次请求。有四个与此功能相关的有效票证：

...还有几个标记为重复项： http://bugs.mysql.com/bug.php?id=19602 、 http://bugs.mysql.com/bug.php?id=19602 、 http://bugs.mysql.com/bug.php?id=13109 ，http://bugs.mysql.com/bug.php?id=14369，http://bugs.mysql.com/bug.php?id=11082 ，http://bugs.mysql.com/bug.php?id=42870，http://bugs.mysql.com/bug.php?id=38137 ，http://bugs.mysql.com/bug.php?id=38137，http://bugs.mysql.com/bug.php?id=30729

如果您对此问题感兴趣，请转到四个有效票证，单击“影响我”按钮，并添加评论，询问是否有人正在研究此功能。

这将增加可见性，并增加实施的可能性。

更新 - 我在 2013 年 12 月发布了此评论。当时版本是 5.7。截至 2023 年 8 月，最新版本为 8.1，CREATE VIEW 文档页面没有变化。

Answer 2

我有类似的需求，我在 MySQL 中解决这个问题的一种方法是在用作文档的

WHERE

子句中添加一个 true 谓词。我承认这很糟糕，但是您是否同意任何文档都比没有文档好？一旦以这种方式进行评论的良好副作用将持续存在

mysqldump

。据我所知，优化器不会受到额外的 true 谓词的阻碍。

视图创建示例：

CREATE OR REPLACE VIEW high_value_employees AS
SELECT *
FROM `employees`
WHERE salary >= 200000
AND 'comment' != 'This view was made by Josh at the request of an important VP who wanted a concise list of who we might be overpaying. Last modified on 26 July 2019.';

然后查看文档...

> SHOW CREATE TABLE high_value_employees \G
*************************** 1. row ***************************
                View: high_value_employees
         Create View: CREATE ALGORITHM=UNDEFINED DEFINER=`jhuber`@`%` SQL SECURITY 
DEFINER VIEW `high_value_employees` AS select `employees`.`salary` AS `salary` from
`employees` where ((`employees`.`salary` >= 200000) and ('comment' <> 'This view was
made by Josh at the request of an important VP who wanted a concise list of who we
might be overpaying. Last modified on 26 July 2019.'))
character_set_client: utf8mb4
collation_connection: utf8mb4_general_ci
1 row in set (0.00 sec)

Answer 3

您可以通过在架构中创建一个表来存储每个视图的注释，从而在视图上创建注释。然后将 information_schema.tables 连接到新表。

-- A view does not show the table-level comments of the underlying table.
-- nor can a view have view-level comments

CREATE TABLE `zztable` (
-- A SQL statement comment. Not stored with the table. Just documents the create table code
  `zz_id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT 'unique primary key. auto increment',
  `zz_descr` varchar(255) NOT NULL COMMENT 'descriptive name. must be unique if not null',
  PRIMARY KEY (`zz_id`),
  UNIQUE KEY `zz_descr_UNIQUE` (`zz_descr`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='a table demonstrating table, column, and view comments. ';

-- select the table from information_schema
SELECT table_type, table_name, table_rows, table_comment
FROM information_schema.tables ta
WHERE ta.table_name LIKE 'zztable%'
ORDER BY ta.table_type, ta.table_name;

-- create a view over the commented table
CREATE OR REPLACE VIEW zztable_vw
AS
SELECT zz_id, zz_descr
FROM zztable;

-- now run the information_schema queries again to see the new view in the results
-- MySQL does not allow view-level comments. 

-- create a new table to contain the view-level comments
CREATE TABLE IF NOT EXISTS `schema_view` (
  `schema_view_id` INT UNSIGNED NOT NULL AUTO_INCREMENT COMMENT 'unique primary key. auto increment. ',
  `schema_view_name` VARCHAR(64) NOT NULL COMMENT 'view name matches information_schema.tables.table_name for VIEW',
  `schema_view_comment` VARCHAR(2048) NULL DEFAULT NULL COMMENT 'the descriptive purpose of the view. ',
  PRIMARY KEY (`schema_view_id`))
ENGINE = InnoDB
COMMENT = 'contains comments for views since MySQL does not store view-level comments. Use this in a join on schema_view_name to information_schema.tables.table_name';

CREATE UNIQUE INDEX `schema_view_name_UNIQUE` ON `schema_view` (`schema_view_name` ASC);

-- insert a view comment
SELECT * FROM schema_view;

INSERT INTO schema_view
(schema_view_name, schema_view_comment)
VALUES ('zztable_vw' , 'a demonstration of documenting view metadata with comments');
COMMIT;

-- modify the query to join to the new schema_view table
-- select the view from information_schema joined to the new table
SELECT ta.table_type, ta.table_name, ta.table_rows, 
    -- show different comments based on table_type
    CASE 
        WHEN ta.table_type = 'BASE TABLE' THEN ta.table_comment
        WHEN ta.table_type = 'VIEW' THEN sv.schema_view_comment
        ELSE NULL
    END AS schema_comment,
    ta.table_comment, 
    sv.schema_view_comment
FROM information_schema.tables ta
-- Show view comments if it exists.
LEFT OUTER JOIN schema_view sv
  ON ta.table_name = sv.schema_view_name
WHERE ta.table_name LIKE 'zztable%'
ORDER BY ta.table_type, ta.table_name;

-- simplify future queries by creating a view
CREATE OR REPLACE VIEW `schema_table_vw`
AS
SELECT ta.table_type, ta.table_name, ta.table_rows, 
    -- show different comments based on type
    CASE 
        WHEN ta.table_type = 'BASE TABLE' THEN ta.table_comment
        WHEN ta.table_type = 'VIEW' THEN sv.schema_view_comment
        ELSE NULL
    END AS schema_comment
FROM information_schema.tables ta
-- Show view comments if it exists.
LEFT OUTER JOIN schema_view sv
  ON ta.table_name = sv.schema_view_name
WHERE ta.table_schema = 'my_schema'
ORDER BY ta.table_type, ta.table_name;

-- 视图级和表级注释现在显示在 schema_comment 中

<table width="200" border="1">
  <tr>
    <th scope="col">table_type</th>
    <th scope="col">table_name</th>
    <th scope="col">table_rows</th>
    <th scope="col">schema_comment</th>
    <th scope="col">table_comment</th>
    <th scope="col">schema_view_comment</th>
  </tr>
  <tr>
    <td>BASE TABLE</td>
    <td>zztable</td>
    <td>0</td>
    <td>a table demonstrating table, column, and view comments.</td>
    <td>a table demonstrating table, column, and view comments.</td>
    <td>NULL</td>
  </tr>
  <tr>
    <td>VIEW</td>
    <td>zztable_vw</td>
    <td>NULL</td>
    <td>a demonstration of documenting view metadata with comments</td>
    <td>VIEW</td>
    <td>a demonstration of documenting view metadata with comments</td>
  </tr>
</table>

Answer 4

我希望能够标记开发中的视图版本，以便我可以跟踪其他环境中的不同版本。我的解决方案很简单：我只需将包含版本的常量字段添加到字段列表中。我喜欢每个检索到的视图记录都附带视图版本的优势。

VIEW `myview` AS
    SELECT 
        '2022-10-11' AS `ViewVersion`,
....

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