This statement creates a new view, or replaces an existing one if the
OR REPLACE clause is given. The select_statement is a
SELECT statement that provides the definition of the view.
The statement can select from base tables or other views.

This statement requires the CREATE VIEW privilege for the view, and
some privilege for each column referred to in the SELECT statement.
If the OR REPLACE clause is present, you must also have the
DELETE privilege for the view.

A view belongs to a database.  By default, a new view is created in the
current 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;

Tables and views share the same namespace within a database, so a database
cannot contain a table and a view that have the same name.

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.

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.

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:


 --- The SELECT statement cannot contain a subquery in the FROM
clause.

 --- The SELECT statement cannot refer to system or user variables.

 --- Within a stored procedure, the definition cannot refer to procedure
parameters or local variables.

 --- 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.  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.

 --- The tables named in the view definition must already exist.

 --- You cannot associate a trigger with a view.


ORDER BY is allowed 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 you get from the
view:

mysql> CREATE VIEW v AS SELECT CHARSET(CHAR(65)), COLLATION(CHAR(65));
Query OK, 0 rows affected (0.00 sec)

mysql> SET NAMES 'latin1';
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT * FROM v;
+-------------------+---------------------+
| CHARSET(CHAR(65)) | COLLATION(CHAR(65)) |
+-------------------+---------------------+
| latin1            | latin1_swedish_ci   |
+-------------------+---------------------+
1 row in set (0.00 sec)

mysql> SET NAMES 'utf8';
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT * FROM v;
+-------------------+---------------------+
| CHARSET(CHAR(65)) | COLLATION(CHAR(65)) |
+-------------------+---------------------+
| utf8              | utf8_general_ci     |
+-------------------+---------------------+
1 row in set (0.00 sec)

The optional ALGORITHM clause is a MySQL extension to standard SQL.
ALGORITHM takes three values: MERGE, TEMPTABLE, or
UNDEFINED. The default algorithm is UNDEFINED if no
ALGORITHM clause is present.  The algorithm affects how MySQL
processes the view.

For MERGE, the text of a statement that refers to the view and the view
definition are merged such that parts of the view definition replace
corresponding parts of the statement.

For TEMPTABLE, the results from the view are retrieved into a
temporary table, which then is used to execute the statement.

For UNDEFINED, MySQL chooses which algorithm to use.  It prefers
MERGE over TEMPTABLE if possible, because MERGE is
usually more efficient and because a view cannot be updatable if a temporary
table is used.

A reason to choose TEMPTABLE explicitly is that locks can be released
on underlying tables after the temporary table has been created and before
it is used to finish processing the statement.  This might result in quicker
lock release than the MERGE algorithm so that other clients that use
the view are not blocked as long.

A view algorithm can be UNDEFINED three ways:


 --- No ALGORITHM clause is present in the CREATE VIEW statement.

 --- The CREATE VIEW statement has an explicit ALGORITHM = UNDEFINED
clause.

 --- ALGORITHM = MERGE is specified for a view that can be processed only
with a temporary table.  In this case, MySQL generates a warning and sets
the algorithm to UNDEFINED.



As mentioned earlier, MERGE is handled by merging corresponding parts
of a view definition into the statement that refers to the view.  The
following examples briefly illustrate how the MERGE algorithm works.
The examples assume that there is a view v_merge that has this
definition:

CREATE ALGORITHM = MERGE VIEW v_merge (vc1, vc2) AS
SELECT c1, c2 FROM t WHERE c3 > 100;

Example 1: Suppose that we issue this statement:

SELECT * FROM v_merge;

MySQL handles the statement as follows:


 --- v_merge becomes t

 --- * becomes vc1, vc2, which corresponds to c1, c2

 --- The view WHERE clause is added


The resulting statement to be executed becomes:

SELECT c1, c2 FROM t WHERE c3 > 100;

Example 2: Suppose that we issue this statement:

SELECT * FROM v_merge WHERE vc1 < 100;

This statement is handled similarly to the previous one, except that
vc1 < 100 becomes c1 < 100 and the view WHERE clause is
added to the statement WHERE clause using an AND connective
(and parentheses are added to make sure the parts of the clause are executed
with correct precedence).  The resulting statement to be executed becomes:

SELECT c1, c2 FROM t WHERE (c3 > 100) AND (c1 < 100);

Effectively, the statement to be executed has a WHERE clause of this
form:

WHERE (select WHERE) AND (view WHERE)

The MERGE algorithm requires a one-to relationship between the rows
in the view and the rows in the underlying table. If this relationship does
not hold, a temporary table must be used instead.  Lack of a one-to-one
relationship occurs if the view contains any of a number of constructs:


 --- Aggregate functions (SUM(), MIN(), MAX(),
COUNT(), and so forth)

 --- DISTINCT

 --- GROUP BY

 --- HAVING

 --- UNION or UNION ALL

 --- Refers only to literal values (in this case, there is no underlying table)


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
relationship between the rows in the view and the rows in the underlying
table.  There are also certain other constructs that make a view
non-updatable. To be more specific, a view is not updatable if it contains
any of the following:


 --- Aggregate functions (SUM(), MIN(), MAX(),
COUNT(), and so forth)

 --- DISTINCT

 --- GROUP BY

 --- HAVING

 --- UNION or UNION ALL

 --- Subquery in the select list

 --- Join

 --- Non-updatable view in the FROM clause

 --- A subquery in the WHERE clause that refers to a table in the
FROM clause

 --- Refers only to literal values (in this case, there is no underlying table to
update)

 --- ALGORITHM = TEMPTABLE (use of a temporary table always makes a view
non-updatable)


With respect to insertability (being updatable with INSERT
statements), an updatable view is insertable if it also satisfies these
additional requirements for the view columns:


 --- There must be no duplicate view column names.

 --- The view must contain all columns in the base table that do not have a
default value.

 --- The view columns must be simple column references and not derived columns.
A derived column is one that is not a simple column reference but is derived
from an expression.  These are examples of derived columns:

3.14159
col1 + 3
UPPER(col2)
col3 / col4
(subquery)


A view that has a mix of simple column references and derived columns is not
insertable, but it can be updatable if you update only those columns that
are not derived.  Consider this view:

CREATE VIEW v AS SELECT col1, 1 AS col2 FROM t;

This view is not insertable because col2 is derived from an
expression. But it is updatable if the update does not try to update
col2.  This update is allowable:

UPDATE v SET col1 = 0;

This update is not allowable because it attempts to update a derived column:

UPDATE v SET col2 = 0;

It is sometimes possible for a multiple-table view to be updatable, assuming
that it can be processed with the MERGE algorithm.  For this to work,
the view must use an inner join (not an outer join or a UNION). Also,
only a single table in the view definition can be updated, so the SET
clause must name only columns from one of the tables in the view.  Views
that use UNION ALL are disallowed even though they might be
theoretically updatable, because the implementation uses temporary tables to
process them.


For a multiple-table updatable view, INSERT can work if it inserts into a
single table.  DELETE is not supported.

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.
