how to sort order of LEFT JOIN in SQL query?

Try using MAX with a GROUP BY.

SELECT u.userName, MAX(c.carPrice)FROM users u    LEFT JOIN cars c ON u.id = c.belongsToUserWHERE u.id = 4;GROUP BY u.userName;

Further information on GROUP BY

The group by clause is used to split the selected records into groups based on unique combinations of the group by columns. This then allows us to use aggregate functions (eg. MAX, MIN, SUM, AVG, ...) that will be applied to each group of records in turn. The database will return a single result record for each grouping.

For example, if we have a set of records representing temperatures over time and location in a table like this:

Location   Time    Temperature--------   ----    -----------London     12:00          10.0Bristol    12:00          12.0Glasgow    12:00           5.0London     13:00          14.0Bristol    13:00          13.0Glasgow    13:00           7.0...

Then if we want to find the maximum temperature by location, then we need to split the temperature records into groupings, where each record in a particular group has the same location. We then want to find the maximum temperature of each group. The query to do this would be as follows:

SELECT Location, MAX(Temperature)FROM TemperaturesGROUP BY Location;

Older MySQL versions this is enough:

SELECT    `userName`,    `carPrice`FROM `users`LEFT JOIN (SELECT * FROM `cars` ORDER BY `carPrice`) as `cars`ON cars.belongsToUser=users.idWHERE `id`='4'

Nowdays, if you use MariaDB the subquery should be limited.

SELECT    `userName`,    `carPrice`FROM `users`LEFT JOIN (SELECT * FROM `cars` ORDER BY `carPrice` LIMIT 18446744073709551615) as `cars`ON cars.belongsToUser=users.idWHERE `id`='4'

Several other answer give the solution using MAX. In some scenarios using an agregate function is either not possilbe, or not performant.

The alternative that I use a lot is to use a correlated sub-query in the join...

SELECT   `userName`,   `carPrice`FROM `users`LEFT JOIN `cars`ON cars.id = (  SELECT id FROM `cars` WHERE BelongsToUser = users.id ORDER BY carPrice DESC LIMIT 1)WHERE `id`='4'