MySQL/MariaDB数据库的视图(VIEW)
作者：尹正杰
版权声明：原创作品，谢绝转载！否则将追究法律责任。
一.视图概述
1>.什么是视图
视图就是一个虚拟的表，保存有实表的查询结果。换句话说，视图并不存储数据，视图的数据来自于实体表(基表)。
视图中的数据事实上存储于“基表”中，因此，其修改操作也会针对基表实现；其修改操作受基表限制。
优点:
1>.将繁琐的查询语句定义为视图，便于下此调用时方便;
2>.视图可以隐藏表结构,尤其是比较铭感的数据(财务薪资查询等);
缺点:
1>.由于视图本身不保存数据，因此对视图的操作会直接将"基表"数据修改啦;
视图的应用场景:
1>.供用户查询(一般情况不建议对视图进行增删改,因为修改视图其本质是对"基表"的修改，如果视图来自多张表，直接对视图进行增删改可能会报错);
2>.创建方法
MariaDB [yinzhengjie]> HELP CREATE VIEW Name: 'CREATE VIEW' Description: Syntax: CREATE [OR REPLACE] [ALGORITHM = {UNDEFINED | MERGE | TEMPTABLE}] [DEFINER = { user | CURRENT_USER }] [SQL SECURITY { DEFINER | INVOKER }] VIEW view_name [(column_list)] AS select_statement [WITH [CASCADED | LOCAL] CHECK OPTION] The CREATE VIEW statement creates a new view, or replaces an existing one if the OR REPLACE clause is given. If the view does not exist, CREATE OR REPLACE VIEW is the same as CREATE VIEW. If the view does exist, CREATE OR REPLACE VIEW is the same as ALTER VIEW. The select_statement is a SELECT statement that provides the definition of the view. (When you select from the view, you select in effect using the SELECT statement.) select_statement can select from base tables or other views. The view definition is "frozen" at creation time, so changes to the underlying tables afterward do not affect the view definition. For example, if a view is defined as SELECT * on a table, new columns added to the table later do not become part of the view. The ALGORITHM clause affects how MySQL processes the view. The DEFINER and SQL SECURITY clauses specify the security context to be used when checking access privileges at view invocation time. The WITH CHECK OPTION clause can be given to constrain inserts or updates to rows in tables referenced by the view. These clauses are described later in this section. The CREATE VIEW statement requires the CREATE VIEW privilege for the view, and some privilege for each column selected by the SELECT statement. For columns used elsewhere in the SELECT statement you must have the SELECT privilege. If the OR REPLACE clause is present, you must also have the DROP privilege for the view. CREATE VIEW might also require the SUPER privilege, depending on the DEFINER value, as described later in this section. When a view is referenced, privilege checking occurs as described later in this section. A view belongs to a database. By default, a new view is created in the default database. To create the view explicitly in a given database, specify the name as db_name.view_name when you create it: MariaDB> CREATE VIEW test.v AS SELECT * FROM t; Within a database, base tables and views share the same namespace, so a base table and a view cannot have the same name. Columns retrieved by the SELECT statement can be simple references to table columns. They can also be expressions that use functions, constant values, operators, and so forth. Views must have unique column names with no duplicates, just like base tables. By default, the names of the columns retrieved by the SELECT statement are used for the view column names. To define explicit names for the view columns, the optional column_list clause can be given as a list of comma-separated identifiers. The number of names in column_list must be the same as the number of columns retrieved by the SELECT statement. Unqualified table or view names in the SELECT statement are interpreted with respect to the default database. A view can refer to tables or views in other databases by qualifying the table or view name with the proper database name. A view can be created from many kinds of SELECT statements. It can refer to base tables or other views. It can use joins, UNION, and subqueries. The SELECT need not even refer to any tables. The following example defines a view that selects two columns from another table, as well as an expression calculated from those columns: MariaDB> CREATE TABLE t (qty INT, price INT); MariaDB> INSERT INTO t VALUES(3, 50); MariaDB> CREATE VIEW v AS SELECT qty, price, qty*price AS value FROM t; MariaDB> SELECT * FROM v; +------+-------+-------+ | qty | price | value | +------+-------+-------+ | 3 | 50 | 150 | +------+-------+-------+ A view definition is subject to the following restrictions: o The SELECT statement cannot contain a subquery in the FROM clause. o The SELECT statement cannot refer to system or user variables. o Within a stored program, the definition cannot refer to program parameters or local variables. o The SELECT statement cannot refer to prepared statement parameters. o Any table or view referred to in the definition must exist. However, after a view has been created, it is possible to drop a table or view that the definition refers to. In this case, use of the view results in an error. To check a view definition for problems of this kind, use the CHECK TABLE statement. o The definition cannot refer to a TEMPORARY table, and you cannot create a TEMPORARY view. o Any tables named in the view definition must exist at definition time. o You cannot associate a trigger with a view. o Aliases for column names in the SELECT statement are checked against the maximum column length of 64 characters (not the maximum alias length of 256 characters). ORDER BY is permitted in a view definition, but it is ignored if you select from a view using a statement that has its own ORDER BY. For other options or clauses in the definition, they are added to the options or clauses of the statement that references the view, but the effect is undefined. For example, if a view definition includes a LIMIT clause, and you select from the view using a statement that has its own LIMIT clause, it is undefined which limit applies. This same principle applies to options such as ALL, DISTINCT, or SQL_SMALL_RESULT that follow the SELECT keyword, and to clauses such as INTO, FOR UPDATE, LOCK IN SHARE MODE, and PROCEDURE. If you create a view and then change the query processing environment by changing system variables, that may affect the results that you get from the view: MariaDB> CREATE VIEW v (mycol) AS SELECT 'abc'; Query OK, 0 rows affected (0.01 sec) MariaDB> SET sql_mode = ''; Query OK, 0 rows affected (0.00 sec) MariaDB> SELECT "mycol" FROM v; +-------+ | mycol | +-------+ | mycol | +-------+ 1 row in set (0.01 sec) MariaDB> SET sql_mode = 'ANSI_QUOTES'; Query OK, 0 rows affected (0.00 sec) MariaDB> SELECT "mycol" FROM v; +-------+ | mycol | +-------+ | abc | +-------+ 1 row in set (0.00 sec) The DEFINER and SQL SECURITY clauses determine which MySQL account to use when checking access privileges for the view when a statement is executed that references the view. The valid SQL SECURITY characteristic values are DEFINER and INVOKER. These indicate that the required privileges must be held by the user who defined or invoked the view, respectively. The default SQL SECURITY value is DEFINER. If a user value is given for the DEFINER clause, it should be a MySQL account specified as 'user_name'@'host_name' (the same format used in the GRANT statement), CURRENT_USER, or CURRENT_USER(). The default DEFINER value is the user who executes the CREATE VIEW statement. This is the same as specifying DEFINER = CURRENT_USER explicitly. If you specify the DEFINER clause, these rules determine the valid DEFINER user values: o If you do not have the SUPER privilege, the only valid user value is your own account, either specified literally or by using CURRENT_USER. You cannot set the definer to some other account. o If you have the SUPER privilege, you can specify any syntactically valid account name. If the account does not actually exist, a warning is generated. o Although it is possible to create a view with a nonexistent DEFINER account, an error occurs when the view is referenced if the SQL SECURITY value is DEFINER but the definer account does not exist. For more information about view security, see https://mariadb.com/kb/en/stored-routine-privileges/. Within a view definition, CURRENT_USER returns the view's DEFINER value by default. For views defined with the SQL SECURITY INVOKER characteristic, CURRENT_USER returns the account for the view's invoker. For information about user auditing within views, see http://dev.mysql.com/doc/refman/5.5/en/account-activity-auditing.html. Within a stored routine that is defined with the SQL SECURITY DEFINER characteristic, CURRENT_USER returns the routine's DEFINER value. This also affects a view defined within such a routine, if the view definition contains a DEFINER value of CURRENT_USER. View privileges are checked like this: o At view definition time, the view creator must have the privileges needed to use the top-level objects accessed by the view. For example, if the view definition refers to table columns, the creator must have some privilege for each column in the select list of the definition, and the SELECT privilege for each column used elsewhere in the definition. If the definition refers to a stored function, only the privileges needed to invoke the function can be checked. The privileges required at function invocation time can be checked only as it executes: For different invocations, different execution paths within the function might be taken. o The user who references a view must have appropriate privileges to access it (SELECT to select from it, INSERT to insert into it, and so forth.) o When a view has been referenced, privileges for objects accessed by the view are checked against the privileges held by the view DEFINER account or invoker, depending on whether the SQL SECURITY characteristic is DEFINER or INVOKER, respectively. o If reference to a view causes execution of a stored function, privilege checking for statements executed within the function depend on whether the function SQL SECURITY characteristic is DEFINER or INVOKER. If the security characteristic is DEFINER, the function runs with the privileges of the DEFINER account. If the characteristic is INVOKER, the function runs with the privileges determined by the view's SQL SECURITY characteristic. Example: A view might depend on a stored function, and that function might invoke other stored routines. For example, the following view invokes a stored function f(): CREATE VIEW v AS SELECT * FROM t WHERE t.id = f(t.name); Suppose that f() contains a statement such as this: IF name IS NULL then CALL p1(); ELSE CALL p2(); END IF; The privileges required for executing statements within f() need to be checked when f() executes. This might mean that privileges are needed for p1() or p2(), depending on the execution path within f(). Those privileges must be checked at runtime, and the user who must possess the privileges is determined by the SQL SECURITY values of the view v and the function f(). The DEFINER and SQL SECURITY clauses for views are extensions to standard SQL. In standard SQL, views are handled using the rules for SQL SECURITY DEFINER. The standard says that the definer of the view, which is the same as the owner of the view's schema, gets applicable privileges on the view (for example, SELECT) and may grant them. MySQL has no concept of a schema "owner", so MySQL adds a clause to identify the definer. The DEFINER clause is an extension where the intent is to have what the standard has; that is, a permanent record of who defined the view. This is why the default DEFINER value is the account of the view creator. The optional ALGORITHM clause is a MySQL extension to standard SQL. It affects how MySQL processes the view. ALGORITHM takes three values: MERGE, TEMPTABLE, or UNDEFINED. The default algorithm is UNDEFINED if no ALGORITHM clause is present. For more information, see https://mariadb.com/kb/en/view-algorithms/. Some views are updatable. That is, you can use them in statements such as UPDATE, DELETE, or INSERT to update the contents of the underlying table. For a view to be updatable, there must be a one-to-one relationship between the rows in the view and the rows in the underlying table. There are also certain other constructs that make a view nonupdatable. The WITH CHECK OPTION clause can be given for an updatable view to prevent inserts or updates to rows except those for which the WHERE clause in the select_statement is true. In a WITH CHECK OPTION clause for an updatable view, the LOCAL and CASCADED keywords determine the scope of check testing when the view is defined in terms of another view. The LOCAL keyword restricts the CHECK OPTION only to the view being defined. CASCADED causes the checks for underlying views to be evaluated as well. When neither keyword is given, the default is CASCADED. For more information about updatable views and the WITH CHECK OPTION clause, see https://mariadb.com/kb/en/inserting-and-updating-with-views/. URL: https://mariadb.com/kb/en/create-view/ MariaDB [yinzhengjie]>
二.实操案例
1>.创建视图
MariaDB [yinzhengjie]> SHOW TABLE STATUS LIKE 'students'G #创建视图之前查看基表状态 *************************** 1. row *************************** Name: students Engine: InnoDB Version: 10 Row_format: Dynamic Rows: 25 Avg_row_length: 655 Data_length: 16384 Max_data_length: 0 Index_length: 0 Data_free: 0 Auto_increment: 26 Create_time: 2019-10-27 17:15:07 Update_time: NULL Check_time: NULL Collation: utf8_general_ci Checksum: NULL Create_options: Comment: row in set (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SHOW TABLES; +-----------------------+ | Tables_in_yinzhengjie | +-----------------------+ | classes | | coc | | courses | | emp | | scores | | students | | teachers | | toc | +-----------------------+ 8 rows in set (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> CREATE VIEW v_students AS SELECT stuid,name,age FROM students; #创建一个名称为"v_students"的视图 Query OK, 0 rows affected (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> SHOW TABLES; +-----------------------+ | Tables_in_yinzhengjie | +-----------------------+ | classes | | coc | | courses | | emp | | scores | | students | | teachers | | toc | | v_students | +-----------------------+ 9 rows in set (0.00 sec) MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SHOW TABLE STATUS LIKE 'v_students'G #查看视图虚表的状态 *************************** 1. row *************************** Name: v_students Engine: NULL Version: NULL Row_format: NULL Rows: NULL Avg_row_length: NULL Data_length: NULL Max_data_length: NULL Index_length: NULL Data_free: NULL Auto_increment: NULL Create_time: NULL Update_time: NULL Check_time: NULL Collation: NULL Checksum: NULL Create_options: NULL Comment: VIEW #注意，看这里的注释是一个视图哟 1 row in set (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SELECT * FROM v_students; #不难发现视图就是一个"基表"通过SELECT命令查询的结果，可以将比较麻烦的SQL语句定义成视图，这样每次查询的时候就不用写一长串啦！ +-------+---------------+-----+ | stuid | name | age | +-------+---------------+-----+ | 1 | Shi Zhongyu | 22 | | 2 | Shi Potian | 22 | | 3 | Xie Yanke | 53 | | 4 | Ding Dian | 32 | | 5 | Yu Yutong | 26 | | 6 | Shi Qing | 46 | | 7 | Xi Ren | 19 | | 8 | Lin Daiyu | 17 | | 9 | Ren Yingying | 20 | | 10 | Yue Lingshan | 19 | | 11 | Yuan Chengzhi | 23 | | 12 | Wen Qingqing | 19 | | 13 | Tian Boguang | 33 | | 14 | Lu Wushuang | 17 | | 15 | Duan Yu | 19 | | 16 | Xu Zhu | 21 | | 17 | Lin Chong | 25 | | 18 | Hua Rong | 23 | | 19 | Xue Baochai | 18 | | 20 | Diao Chan | 19 | | 21 | Huang Yueying | 22 | | 22 | Xiao Qiao | 20 | | 23 | Ma Chao | 23 | | 24 | Xu Xian | 27 | | 25 | Sun Dasheng | 100 | +-------+---------------+-----+ rows in set (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> SELECT stuid,name,age FROM students; +-------+---------------+-----+ | stuid | name | age | +-------+---------------+-----+ | 1 | Shi Zhongyu | 22 | | 2 | Shi Potian | 22 | | 3 | Xie Yanke | 53 | | 4 | Ding Dian | 32 | | 5 | Yu Yutong | 26 | | 6 | Shi Qing | 46 | | 7 | Xi Ren | 19 | | 8 | Lin Daiyu | 17 | | 9 | Ren Yingying | 20 | | 10 | Yue Lingshan | 19 | | 11 | Yuan Chengzhi | 23 | | 12 | Wen Qingqing | 19 | | 13 | Tian Boguang | 33 | | 14 | Lu Wushuang | 17 | | 15 | Duan Yu | 19 | | 16 | Xu Zhu | 21 | | 17 | Lin Chong | 25 | | 18 | Hua Rong | 23 | | 19 | Xue Baochai | 18 | | 20 | Diao Chan | 19 | | 21 | Huang Yueying | 22 | | 22 | Xiao Qiao | 20 | | 23 | Ma Chao | 23 | | 24 | Xu Xian | 27 | | 25 | Sun Dasheng | 100 | +-------+---------------+-----+ rows in set (0.00 sec) MariaDB [yinzhengjie]>
[root@node105.yinzhengjie.org.cn ~]# ll /mysql/3306/data/yinzhengjie/ total 808 -rw-rw---- 1 mysql mysql 1277 Oct 27 17:15 classes.frm -rw-rw---- 1 mysql mysql 98304 Oct 27 17:15 classes.ibd -rw-rw---- 1 mysql mysql 976 Oct 27 17:15 coc.frm -rw-rw---- 1 mysql mysql 98304 Oct 27 17:15 coc.ibd -rw-rw---- 1 mysql mysql 1251 Oct 27 17:15 courses.frm -rw-rw---- 1 mysql mysql 98304 Oct 27 17:15 courses.ibd -rw-rw---- 1 mysql mysql 61 Oct 27 17:15 db.opt -rw-rw---- 1 mysql mysql 494 Oct 28 06:30 emp.frm -rw-rw---- 1 mysql mysql 98304 Oct 28 06:39 emp.ibd -rw-rw---- 1 mysql mysql 1001 Oct 27 17:15 scores.frm -rw-rw---- 1 mysql mysql 98304 Oct 27 17:15 scores.ibd -rw-rw---- 1 mysql mysql 1208 Oct 27 17:15 students.frm -rw-rw---- 1 mysql mysql 98304 Oct 27 17:15 students.ibd -rw-rw---- 1 mysql mysql 1298 Oct 27 17:15 teachers.frm -rw-rw---- 1 mysql mysql 98304 Oct 27 17:15 teachers.ibd -rw-rw---- 1 mysql mysql 973 Oct 27 17:15 toc.frm -rw-rw---- 1 mysql mysql 98304 Oct 27 17:15 toc.ibd -rw-rw---- 1 mysql mysql 660 Oct 28 07:40 v_students.frm [root@node105.yinzhengjie.org.cn ~]# [root@node105.yinzhengjie.org.cn ~]# ll /mysql/3306/data/yinzhengjie/ | grep v_students; #视图只有表结构存储文件并没有单独的数据存储文件。 -rw-rw---- 1 mysql mysql 660 Oct 28 07:40 v_students.frm [root@node105.yinzhengjie.org.cn ~]#
2>.查看视图定义
MariaDB [yinzhengjie]> SHOW CREATE VIEW v_students; +------------+----------------------------------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------------+----------------------+----------------------+| View | Create View | character_set_client | collation_connection |+------------+----------------------------------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------------+----------------------+----------------------+| v_students | CREATE ALGORITHM=UNDEFINED DEFINER=`root`@`localhost` SQL SECURITY DEFINER VIEW `v_students` AS select `students`.`StuID` AS `stui d`,`students`.`Name` AS `name`,`students`.`Age` AS `age` from `students` | utf8mb4 | utf8mb4_general_ci |+------------+----------------------------------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------------+----------------------+----------------------+1 row in set (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> SHOW CREATE VIEW v_studentsG *************************** 1. row *************************** View: v_students Create View: CREATE ALGORITHM=UNDEFINED DEFINER=`root`@`localhost` SQL SECURITY DEFINER VIEW `v_students` AS select `students`.`StuID` A S `stuid`,`students`.`Name` AS `name`,`students`.`Age` AS `age` from `students`character_set_client: utf8mb4 collation_connection: utf8mb4_general_ci 1 row in set (0.00 sec) MariaDB [yinzhengjie]>
3>.删除视图
MariaDB [yinzhengjie]> HELP DROP VIEW Name: 'DROP VIEW' Description: Syntax: DROP VIEW [IF EXISTS] view_name [, view_name] ... [RESTRICT | CASCADE] DROP VIEW removes one or more views. You must have the DROP privilege for each view. If any of the views named in the argument list do not exist, MySQL returns an error indicating by name which nonexisting views it was unable to drop, but it also drops all of the views in the list that do exist. The IF EXISTS clause prevents an error from occurring for views that don't exist. When this clause is given, a NOTE is generated for each nonexistent view. See [HELP SHOW WARNINGS]. RESTRICT and CASCADE, if given, are parsed and ignored. URL: https://mariadb.com/kb/en/drop-view/ MariaDB [yinzhengjie]> MariaDB [yinzhengjie]>
MariaDB [yinzhengjie]> SHOW TABLES; +-----------------------+ | Tables_in_yinzhengjie | +-----------------------+ | classes | | coc | | courses | | emp | | scores | | students | | teachers | | toc | | v_students | +-----------------------+ 9 rows in set (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> DROP VIEW v_students; #删除名为"v_students"的视图 Query OK, 0 rows affected (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]> SHOW TABLES; +-----------------------+ | Tables_in_yinzhengjie | +-----------------------+ | classes | | coc | | courses | | emp | | scores | | students | | teachers | | toc | +-----------------------+ 8 rows in set (0.00 sec) MariaDB [yinzhengjie]> MariaDB [yinzhengjie]>