Benefiting from the Relational System - Mastering phpMyAdmin 3.4 for Effective MySQL Management - page 185

In the possible choices for the related key, we see the keys defined in all the InnoDB
tables of the same database. (Creating a cross-database relation is currently not
supported in phpMyAdmin.) The keys defined in the current table are also shown,
as self-referring relations are possible. Let us remove the internal relation for the
author_id column and click on Save . Our goal is to add an InnoDB -type relation for
the author_id column, but it's not possible as the No index defined! message appears
on this line. This is because foreign key definitions in InnoDB or PBXT can be done
only if both the columns have indexes.
Other conditions regarding constraints are explained in the
MySQL manual. Please refer to http://dev.mysql.com/doc/
refman/5.1/en/innodb-foreign-key-constraints.html .
Thus, we come back to the Structure page for the book table and add an ordinary
(non-unique) index to the author_id column producing the following screen:
In the Relation view , we can try again to add the relation we wanted; it works
this time!