I was always curious but can’t find a concise answer. Any help?
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It cannot have more than one primary key. It can, however, have more than one column in the primary key. It can also have more than one unique index. Generally, the unique index is the primary key, but it is possible to have multiple unique indices on a single table. Off the top of my head I cannot think of an example, but when I do I will add it.
EDIT: Maybe this: In the US, the Department of Motor Vehicles might have a person table with 2 unique columns — Social Security Number and Driver’s License Number. Both should be unique.