Mis juhtub INNER JOIN-is, kui üks rida vasakul vastab mitmele realt paremal?

In the case of INNER JOIN, if one row from the left table matches multiple rows from the right table, 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, corresponding 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 left rows to all matching right rows.