Oracle 5.0 Reference Manual page 1054

Prefixes can be up to 1000 bytes long (767 bytes for
measured in bytes, whereas the prefix length in
number of characters for nonbinary data types (CHAR, VARCHAR, TEXT). Take this into account when
specifying a prefix length for a column that uses a multi-byte character set.
• An
index_col_name
future extensions for specifying ascending or descending index value storage. Currently, they are
parsed but ignored; index values are always stored in ascending order.
• When you use
ORDER BY
the initial number of bytes indicated by the
• You can create special
storage engine supports
and columns. Indexing always happens over the entire column; column prefix indexing is
TEXT
not supported and any prefix length is ignored if specified. See
Functions", for details of operation.
• You can create
SPATIAL
tables and indexed columns must be declared as
MyISAM
Extensions".
• tables support checking of foreign key constraints. The columns of the referenced table
InnoDB
must always be explicitly named. Both
more detailed information and examples, see
For information specific to foreign keys in InnoDB, see
Constraints".
KEY
For other storage engines, MySQL Server parses and ignores the
syntax in
CREATE TABLE
engines. See Section 1.8.5.4, "Foreign
Important
For users familiar with the ANSI/ISO SQL Standard, please note that no
storage engine, including InnoDB, recognizes or enforces the
used in referential integrity constraint definitions. Use of an explicit
clause will not have the specified effect, and also causes
UPDATE
be avoided.
The
composite (multiple-column) foreign key are handled when comparing to a
primary key.
SIMPLE, which permit a foreign key to be all or partially NULL. In that case,
the (child table) row containing such a foreign key is permitted to be inserted,
and does not match any row in the referenced (parent) table. It is possible to
implement other semantics using triggers.
Additionally, MySQL requires that the referenced columns be indexed for
performance. However,
referenced columns be declared
foreign key references to nonunique keys or keys that contain
not well defined for operations such as
are advised to use foreign keys that reference only keys that are both
(or PRIMARY) and
MySQL does not recognize or support "inline
(as defined in the SQL standard) where the references are defined as part of
CREATE TABLE
specification can end with
or on a column in a SELECT, the server sorts values using only
GROUP BY
max_sort_length
indexes, which are used for full-text searches. Only the
FULLTEXT
indexes. They can be created only from CHAR, VARCHAR,
FULLTEXT
indexes on spatial data types. Spatial types are supported only for
ON DELETE
statements. The
CHECK
Keys".
clauses to be ignored. For these reasons, specifying
clause in the SQL standard controls how
MATCH
essentially implements the semantics defined by
InnoDB
InnoDB
NOT NULL.
1034
Syntax
tables). Note that prefix limits are
InnoDB
statements is interpreted as
CREATE TABLE
or DESC. These keywords are permitted for
ASC
[471]
Section 12.9, "Full-Text Search
NULL. See
NOT
and
ON UPDATE
Section 13.1.10.2, "Using
Section 14.2.3.4,
FOREIGN KEY
clause is parsed but ignored by all storage
does not enforce any requirement that the
or
UNIQUE NOT
or
UPDATE
REFERENCES
system variable.
MyISAM
Section 12.16, "Spatial
actions on foreign keys. For
Constraints".
FOREIGN KEY
"InnoDB and
FOREIGN
and
REFERENCES
clause
MATCH
MATCH
and
ON DELETE
should
MATCH
values in a
NULL
MATCH
NULL. The handling of
values is
NULL
CASCADE. You
DELETE
UNIQUE
specifications"
ON