In a left join, does the join key matter? If the wrong join key is used, will it produce incorrect data?

In a left join, the join key is critical because it determines which rows from the left table are matched to rows in the right table. If you use an incorrect key, rows that should match will not, leading to missing or duplicate data, and the unmatched rows will still appear with NULLs from the right side. Therefore, using the wrong join key will produce incorrect results.