13.1.21 CREATE VIEW Syntax

The CREATE VIEW statement creates a new view, or replaces an existing view if the OR REPLACE clause is given. If the view dose 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.

For information about restrictions on view use, see Section C.5, "Restrictions on Views".

The select_statement is a SELECT statement that provides the definition of the view.(Selecting from the view selects, in effect, using the SELECT statement.) The select_statement can select from base tables or other views.

The view definition is "frozen" at creation time and is not affected by subsequent changes to the definitions of the underlying tables. 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, add columns dropped from the table will result in an error when selecting from 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 is 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, use db_name.view_name syntax to qualify the view name with the database name:

CREATE VIEW test.v AS SELECT * FROM t;

Unqualified table or view names in the SELECT statement are also 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 appropriate database name.

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, or expressions that use functions, constant values, operators, and so forth.

A view must have unique column names with no duplicates, just like a base table. 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, specify the optional column_list clause 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.

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:

CREATE VIEW v_today(today) as select current_date;

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;
Query OK, 0 rows affected (0.01 sec)

mysql> SELECT * FROM v;
+------+-------+-------+
| qty  | price | value |
+------+-------+-------+
|    3 |    50 |   150 |
+------+-------+-------+
1 row in set (0.00 sec)

A view definition is subject to the following restrictions:

  • Before MySQL 5.7.7, the SELECT statement cannot contain a subquery in the FROM clause.
  • The SELECT statement cannot refer to system variables or user-defined variables.
  • Within a stored program, the SELECT statement cannot refer to program parameters or local variables.
  • The SELECT statement cannot refer to prepared statement parameter.
  • Any table or view referred to in the definition must exist. If, after the view has been created, table or view that the definition refers to is dropped, use of the view results in an error. To check a view definition for problems of this kind, use the CHECK TABLE statement.
  • The definition cannot refer to a TEMPORARY table, and you cannot create a TEMPORARY view.
  • You cannot associate a trigger with a view.
  • 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.

The results obtained from a view may be affected if you change the query processing environment by changing system variables:

mysql> CREATE VIEW v(mycol) AS SELECT 'abc';
ERROR 1050 (42S01): Table 'v' already exists
mysql> SET sql_mode = '';
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT "mycol" FROM v;
+-------+
| mycol |
+-------+
| mycol |
+-------+
1 row in set (0.00 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. The valid SQL SECURITY characteristic values are DEFINER (the default) and INVOKER. These indicate that the required privileges must be held by the user who defined or invoked the view, respectively.

If a user value is given for the DEFINER clause, it should be a MySQL account specified as 'user_name'@'host_name', 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 the DEFINER clause is present, these rules determine the valid DEFINER user values:

  • 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.

  • If you have the SUPER privilege, you can specify any syntactically valid account name. If the account does not exist, a warning is generated.

  • 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 Section 23.6, "Access Control for Stored Programs and Views"

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 return the account for the view's invoker. For information about user auditing within views, see Section 6.3.12, "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.

MySQL checks view privileges like this:

  • 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.

  • 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.)

  • 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.

  • 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()

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