定义外键的（列类型）

I'm Designing a database using MySQL , Right now am in the phase of documenting in which I have to Define every Column's data type . when it comes to each table on its own , there's no problem , but I have problem with 1-m Relationships , because I don't know how to Define the Foreign key's Data type ( because in its original table its type is "SERIAL" - coz its Primary Key ) But I can't Define it as SERIAL when it's FK . Can You Help Me ?I'm Designing a database using MySQL , Right no