将用户名条件从WHERE子句移动到ON子句.

Move the username condition from the WHERE clause to the ON clause.

SELECT *
FROM cars
LEFT JOIN carLog
  ON cars.id=carLog.car
     AND carLog.username='Juddling'

在JOIN已经完成时，将应用WHERE子句.这意味着，它将丢弃LEFT JOIN添加的NULL行.

The WHERE clause is applied when the JOIN is already completed. This means, it will discard the NULL rows that the LEFT JOIN added.