Cross-schema Foreign Keys Can a foreign key in one MySQL database/schema refer to a table in a different database/schema?

If so, what limitations are there on doing this?
Yes, you just need to specify the schema before the table name. For example, if the referencing table is in schema qa and the referenced table in xx:

ALTER TABLE qa.test_results 
    ADD COLUMN file INTEGER,
    ADD FOREIGN KEY (file) REFERENCES xx.files (file) ;


Then all of your queries will also need to be formatted using schema.table