13.1.12 CREATE VIEW 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. This statement was added in MySQL 5.0.1. 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:

mysql> 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.

Note

Prior to MySQL 5.0.72, when you modify an existing view, the server saves a backup of the current view definition under the view database directory, in a subdirectory named arc. The backup file for a view v is named v.frm-00001. If you alter the view again, the next backup is named v.frm-00002. The three latest view backup definitions are stored.

Backed up view definitions are not preserved by mysqldump, or any other such programs, but you can retain them using a file copy operation. However, they are not needed for anything but to provide you with a backup of your previous view definition.

It is safe to remove these backup definitions, but only while mysqld is not running. If you delete the arc subdirectory or its files while mysqld is running, an error occurs the next time you try to alter the view:

mysql> ALTER VIEW v AS SELECT * FROM t;
ERROR 6 (HY000): Error on delete of '.\test\arc/v.frm-0004' (Errcode:
2)

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:

mysql> CREATE TABLE t (qty INT, price INT);
mysql> INSERT INTO t VALUES(3, 50);
mysql> CREATE VIEW v AS SELECT qty, price, qty*price AS value FROM t;
mysql> SELECT * FROM v;
+------+-------+-------+
| qty  | price | value |
+------+-------+-------+
|    3 |    50 |   150 |
+------+-------+-------+

A view definition is subject to the following restrictions:

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:

mysql> CREATE VIEW v (mycol) AS SELECT 'abc';
Query OK, 0 rows affected (0.01 sec)

mysql> SET sql_mode = '';
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT "mycol" FROM v;
+-------+
| mycol |
+-------+
| mycol |
+-------+
1 row in set (0.01 sec)

mysql> SET sql_mode = 'ANSI_QUOTES';
Query OK, 0 rows affected (0.00 sec)

mysql> 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. These clauses were addded in MySQL 5.0.13, but have no effect until MySQL 5.0.16. The legal 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 legal DEFINER user values:

For more information about view security, see Section 18.5, “Access Control for Stored Programs and Views”.

Within a view definition, CURRENT_USER returns the view's DEFINER value by default as of MySQL 5.0.24. For older versions, and 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 Section 6.3.8, “SQL-Based MySQL Account Activity Auditing”.

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.

As of MySQL 5.0.16 (when the DEFINER and SQL SECURITY clauses were implemented), view privileges are checked like this:

Prior to MySQL 5.0.16 (before the DEFINER and SQL SECURITY clauses were implemented), privileges required for objects used in a view are checked at view creation time.

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.

If you invoke a view that was created before MySQL 5.0.13, it is treated as though it was created with a SQL SECURITY DEFINER characteristic and with a DEFINER value that is the same as your account. However, because the actual definer is unknown, MySQL issues a warning. To eliminate the warning, it is sufficient to re-create the view so that the view definition includes a DEFINER clause.

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 Section 18.4.2, “View Processing 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. The WITH CHECK OPTION clause was implemented in MySQL 5.0.2.

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 Section 18.4.3, “Updatable and Insertable Views”.