INNER JOIN

A join matches rows in one table to rows in another, using a condition that is almost always a foreign key meeting a primary key.

JOIN on its own means INNER JOIN: only rows with a match on both sides survive.

Aliases b and a keep it readable. Once two tables are involved, qualifying every column with its table saves you from ambiguity errors later.

✦ An inner join silently drops the unmatched

Bessie Head is in authors and has no books, so she does not appear above. A book with a NULL author_id would vanish the same way. Nothing warns you: the query succeeds and the answer is quietly incomplete. When a count comes back lower than expected, this is the first thing to check.