Is string or int preferred for foreign keys?

I have a user table with userid and username columns, and both are unique.

Between userid and username, which would be better to use as a foreign key and why?
My Boss wants to use string, is that ok?

Solution

Is string or int preferred for foreign keys?

It depends

There are many existing discussions on the trade-offs between Natural and Surrogate Keys - you will need to decide on what works for you, and what the 'standard' is within your organisation.

In the OP's case, there is both a surrogate key (int userId) and a natural key (char or varchar username). Either column can be used as a Primary key for the table, and either way, you will still be able to enforce uniqueness of the other key.

Here are some considerations when choosing one way or the other:

The case for using Surrogate Keys (e.g. UserId INT AUTO_INCREMENT)

If you use a surrogate, (e.g. UserId INT AUTO_INCREMENT) as the Primary Key, then all tables referencing table MyUsers should then use UserId as the Foreign Key.

You can still however enforce uniqueness of the username column through use of an additional unique index, e.g.:

CREATE TABLE `MyUsers` (
  `userId` int NOT NULL AUTO_INCREMENT,
  `username` varchar(100) NOT NULL,
  ... other columns
  PRIMARY KEY(`userId`),
  UNIQUE KEY UQ_UserName (`username`)

As per @Dagon, using a narrow primary key (like an int) has performance and storage benefits over using a wider (and variable length) value like varchar. This benefit also impacts further tables which reference MyUsers, as the foreign key to userid will be narrower (fewer bytes to fetch).

Another benefit of the surrogate integer key is that the username can be changed easily without affecting tables referencing MyUsers. If the username was used as a natural key, and other tables are coupled to MyUsers via username, it makes it very inconvenient to change a username (since the Foreign Key relationship would otherwise be violated). If updating usernames was required on tables using username as the foreign key, a technique like ON UPDATE CASCADE is needed to retain data integrity.

The case for using Natural Keys (i.e. username)

One downside of using Surrogate Keys is that other tables which reference MyUsers via a surrogate key will need to be JOINed back to the MyUsers table if the Username column is required. One of the potential benefits of Natural keys is that if a query requires only the Username column from a table referencing MyUsers, that it need not join back to MyUsers to retrieve the user name, which will save some I/O overhead.