define a RESTRICT  type constraint, and there is a child row with several parent rows, InnoDB  does not permit the deletion of any of the parent rows. 

•If ON UPDATE CASCADE  or ON UPDATE SET NULL  recurses to update the same table  it has previously updated during the same cascade, it acts like RESTRICT . This means that you cannot use self- referential ON UPDATE CASCADE  or ON UPDATE SET NULL  operations. This is to prevent infinite loops resulting from cascaded updates. A self-referential ON DELETE SET NULL , on the other hand, is possible, as is a self-referential ON DELETE CASCADE . Cascading operations may not be nested more than 15 levels deep. 

•In an SQL statement that inserts, deletes, or updates many rows, foreign key constraints (like unique constraints) are checked row-by-row. When performing foreign key checks, InnoDB  sets shared row- level locks on child or parent records that it must examine. MySQL checks foreign key constraints immediately; the check is not deferred to transaction commit. According to the SQL standard, the default behavior should be deferred checking. That is, constraints are only checked after the entire SQL statement  has been processed. This means that it is not possible to delete a row that refers to itself using a foreign key. 

•No storage engine, including InnoDB , recognizes or enforces the MATCH  clause used in referential- integrity constraint definitions. Use of an explicit MATCH  clause does not have the specified effect, and it causes ON DELETE  and ON UPDATE  clauses to be ignored. Specifying the MATCH  should be avoided. 

The MATCH  clause in the SQL standard controls how NULL  values in a composite (multiple-column) foreign key are handled when comparing to a primary key in the referenced table. MySQL essentially implements the semantics defined by MATCH SIMPLE , which permits a foreign key to be all or partially NULL . In that case, a (child table) row containing such a foreign key can be inserted even though it does not match any row in the referenced (parent) table. (It is possible to implement other semantics using triggers.) 

•MySQL requires that the referenced columns be indexed for performance reasons. However, MySQL does not enforce a requirement that the referenced columns be UNIQUE  or be declared NOT NULL . 

A FOREIGN KEY  constraint that references a non- UNIQUE  key is not standard SQL but rather an InnoDB  extension. The NDB  storage engine, on the other hand, requires an explicit unique key (or primary key) on any column referenced as a foreign key. 

The handling of foreign key references to nonunique keys or keys that contain NULL  values is not well defined for operations such as UPDATE  or DELETE CASCADE . You are advised to use foreign keys that reference only UNIQUE  (including PRIMARY ) and NOT NULL  keys. 

•MySQL parses but ignores “inline REFERENCES  specifications”  (as defined in the SQL standard) where the references are defined as part of the column specification. MySQL accepts REFERENCES  clauses only when specified as part of a separate FOREIGN KEY  specification. For storage engines that do not support foreign keys (such as MyISAM ), MySQL Server parses and ignores foreign key specifications. 

For information about foreign key constraints, see Section 13.1.20.5, “FOREIGN KEY Constraints” . 

## 1.7.2.4 ’--’ as the Start of a Comment 

Standard SQL uses the C syntax /* this is a comment */  for comments, and MySQL Server supports this syntax as well. MySQL also support extensions to this syntax that enable MySQL-specific SQL to be embedded in the comment, as described in Section 9.7, “Comments” . 

Standard SQL uses “ -- ”  as a start-comment sequence. MySQL Server uses #  as the start comment character. MySQL Server also supports a variant of the --  comment style. That is, the --  start-comment sequence must be followed by a space (or by a control character such as a newline). The space is required 

