Keys and Relationships

A primary key identifies a row. It is unique and never null, which is what makes a row addressable at all.

Prefer a key with no meaning, such as an auto-assigned id. Anything meaningful, an email, a title, a national number, eventually changes or turns out not to be unique.

A foreign key points at a row in another table. books.author_id holds the id of a row in authors.

That single idea is what makes the model relational. The author's name is stored once, in one row, and every book refers to it. Correct a spelling there and every book is correct, because there is only one copy of the fact.

The join is module 5. For now, notice that the name appears once in the authors table and is reached by every book that points at it.