manual.texi:

@example

FOREIGN KEY (index_col_name, ...)

                  REFERENCES table_name (index_col_name, ...)

                  [ON DELETE CASCADE | ON DELETE SET NULL]

@end example

Both tables have to be InnoDB type and @strong{there must be an index

where the foreign key and the referenced key are listed as the first

columns}. InnoDB does not auto-create indexes on foreign keys or

referenced keys: you have to create them explicitly.

Corresponding columns in the foreign key

and the referenced key must have similar internal data types

inside InnoDB so that they can be compared without a type

conversion.

The size and the signedness of integer types has to be the same.

The length of string types need not be the same.

Starting from version 3.23.50 you can also associate the

@code{ON DELETE CASCADE} or @code{ON DELETE SET NULL}

clause with the foreign key constraint.

If @code{ON DELETE CASCADE} is specified, and a row in the parent

table is deleted, then InnoDB automatically deletes also all those rows

in the child table whose foreign key values are equal to

the referenced key value in the parent row. If @code{ON DELETE SET NULL}

is specified, the child rows are automatiaclly updated so that the

columns in the foreign key are set to the SQL NULL value.

Starting from version 3.23.50, InnoDB does not check foreign key

constraints on those foreign key or referenced key values

which contain a NULL column.

Starting from version 3.23.50 the InnoDB parser allows you to

use backquotes (`) around table and column names in the above

definition.

definition but the InnoDB parser is not yet aware of possible

variable @code{lower_case_table_names} you give in @file{my.cnf}.

An example:

@example

CREATE TABLE parent(id INT NOT NULL, PRIMARY KEY (id)) TYPE=INNODB;

CREATE TABLE child(id INT, parent_id INT, INDEX par_ind (parent_id),

             FOREIGN KEY (parent_id) REFERENCES parent(id)) TYPE=INNODB;

             FOREIGN KEY (parent_id) REFERENCES parent(id)

             ON DELETE SET NULL

) TYPE=INNODB;

@end example

Both tables have to be InnoDB type and @strong{there must be an index

where the foreign key and the referenced key are listed as the first

columns}. InnoDB does not auto-create indexes on foreign keys or

referenced keys: you have to create them explicitly.

If MySQL gives the error number 1005 from a @code{CREATE TABLE}

statement, and the error message string refers to errno 150, then

@end example

Remember to create the required indexes first, though.

Starting from version 3.23.50, InnoDB does not check foreign key

constraints on those foreign key or referenced key values

which contain a NULL column.

In InnoDB versions < 3.23.50 @code{ALTER TABLE}

or @code{CREATE INDEX}

should not be used in connection with tables which have foreign

processed as an @code{ALTER TABLE}, and these

restrictions apply also to it.

Corresponding columns in the foreign key

and the referenced key must have similar internal data types

inside InnoDB so that they can be compared without a type

conversion. The length of string types need not be the same.

The size and the signedness of integer types has to be the same.

When doing foreign key checks InnoDB sets shared row

level locks on child or parent records it has to look at.

InnoDB checks foreign key constraints immediately: the check