If your primary key uses BIGSERIAL, use BIGINT as the type for the foreign key (field that defines the relationship). If using SERIAL, use INT as the type for the field with the relationship. There are two patterns for specifying relationships. First, you can write the reference inline when specifying the table. Alternatively, you can use an ALTER TABLE command to add the relationship. When using the first strategy, the table to which you are making the relationship must already exist (or be specified first in the `.sql` file).