Views
本文档描述了如何在 TiDB 中使用视图。
概述
TiDB 支持视图。视图充当虚拟表,其模式由创建视图的 SELECT
语句定义。
- 你可以创建视图以仅向用户暴露安全的字段和数据,从而确保底层表中敏感字段和数据的安全。
- 你可以为经常使用的复杂查询创建视图,以简化和方便复杂查询的操作。
创建视图
在 TiDB 中,复杂查询可以通过 CREATE VIEW
语句定义为视图。语法如下:
CREATE VIEW view_name AS query;
注意,不能创建与已有视图或表同名的视图。
例如,多表连接查询 通过连接 books
表和 ratings
表,使用 JOIN
语句获取书籍的平均评分列表。
为了后续查询的方便,可以使用以下语句将查询定义为视图:
CREATE VIEW book_with_ratings AS
SELECT b.id AS book_id, ANY_VALUE(b.title) AS book_title, AVG(r.score) AS average_score
FROM books b
LEFT JOIN ratings r ON b.id = r.book_id
GROUP BY b.id;
查询视图
一旦创建了视图,可以像查询普通表一样使用 SELECT
语句查询视图。
SELECT * FROM book_with_ratings LIMIT 10;
当 TiDB 查询视图时,它会查询与视图关联的 SELECT
语句。
更新视图
目前,TiDB 中的视图不支持 ALTER VIEW view_name AS query;
,你可以通过以下两种方式“更新”视图:
- 使用
DROP VIEW view_name;
语句删除旧的视图,然后通过创建新视图的CREATE VIEW view_name AS query;
语句更新视图。 - 使用
CREATE OR REPLACE VIEW view_name AS query;
语句覆盖同名的已有视图。
CREATE OR REPLACE VIEW book_with_ratings AS
SELECT b.id AS book_id, ANY_VALUE(b.title), ANY_VALUE(b.published_at) AS book_title, AVG(r.score) AS average_score
FROM books b
LEFT JOIN ratings r ON b.id = r.book_id
GROUP BY b.id;
获取视图相关信息
使用 SHOW CREATE TABLE|VIEW view_name
语句
SHOW CREATE VIEW book_with_ratings\G
结果如下:
*************************** 1. row ***************************
View: book_with_ratings
Create View: CREATE ALGORITHM=UNDEFINED DEFINER=`root`@`%` SQL SECURITY DEFINER VIEW `book_with_ratings` (`book_id`, `ANY_VALUE(b.title)`, `book_title`, `average_score`) AS SELECT `b`.`id` AS `book_id`,ANY_VALUE(`b`.`title`) AS `ANY_VALUE(b.title)`,ANY_VALUE(`b`.`published_at`) AS `book_title`,AVG(`r`.`score`) AS `average_score` FROM `bookshop`.`books` AS `b` LEFT JOIN `bookshop`.`ratings` AS `r` ON `b`.`id`=`r`.`book_id` GROUP BY `b`.`id`
character_set_client: utf8mb4
collation_connection: utf8mb4_general_ci
1 row in set (0.00 sec)
查询 INFORMATION_SCHEMA.VIEWS
表
SELECT * FROM information_schema.views WHERE TABLE_NAME = 'book_with_ratings'\G
结果如下:
*************************** 1. row ***************************
TABLE_CATALOG: def
TABLE_SCHEMA: bookshop
TABLE_NAME: book_with_ratings
VIEW_DEFINITION: SELECT `b`.`id` AS `book_id`,ANY_VALUE(`b`.`title`) AS `ANY_VALUE(b.title)`,ANY_VALUE(`b`.`published_at`) AS `book_title`,AVG(`r`.`score`) AS `average_score` FROM `bookshop`.`books` AS `b` LEFT JOIN `bookshop`.`ratings` AS `r` ON `b`.`id`=`r`.`book_id` GROUP BY `b`.`id`
CHECK_OPTION: CASCADED
IS_UPDATABLE: NO
DEFINER: root@%
SECURITY_TYPE: DEFINER
CHARACTER_SET_CLIENT: utf8mb4
COLLATION_CONNECTION: utf8mb4_general_ci
1 row in set (0.00 sec)
删除视图
使用 DROP VIEW view_name;
语句删除视图。
DROP VIEW book_with_ratings;
限制
关于 TiDB 中视图的限制,请参见 Limitations of Views。