What happens in INNER JOIN if one row on the left matches multiple rows on the right?

In the case of INNER JOIN, if one row from the left table corresponds to multiple rows from the right, the result will be multiple rows — each combination of the left row with each matching right row. That is, for each row on the left, new rows are created in the result, equal to the number of matches on the right.

Example:

-- Table A
id | name
1  | Alice

-- Table B
id | hobby
1  | Chess
1  | Painting

SELECT A.name, B.hobby FROM A INNER JOIN B ON A.id = B.id;

Result:

name	hobby
Alice	Chess
Alice	Painting

Thus, INNER JOIN expands the rows on the left to all corresponding rows on the right.