Syntax variants (which ones we'll do?):

CREATE USER xxx DEFAULT ROLE yyy;

ALTER USER xxx DEFAULT ROLE yyy;

SET DEFAULT ROLE yyy;

SET DEFAULT ROLE yyy [ FOR xxx ];

When a default role is set, the server implicitly runs SET ROLE yyy for every new connection (or after a COM_CHANGE_USER for a user xxx).

The default role is stored in a new column in the mysql.user table.