MySQL的加入三表，如果一个表中没有记录

Question

$numQuery1 = " 
SELECT d.* 
    , r.* 
    , AVG(c.on_time)+AVG(c.friendly)+AVG(c.language_skills)+AVG(c.professional) ranking 
    FROM comment c 
    LEFT 
    JOIN driver d 
    ON d.userid = c.driver_id 
    LEFT 
    JOIN driver_rental r 
    ON r.email = d.email 
WHERE 
    ( 
     $driver_rental IS NOT NULL 
    AND $driver_rental != '' 
    AND car_type_1 >= $_SESSION[car_type_1] 
    ) 
    OR car_type_2>=$_SESSION[car_type_1]); 
"; 

如果评语表有没有driver_id记录，结果是空的，我可以忽略评语表，但在select语句不能没有它？

Answer 1

$numQuery1 = "SELECT driver.*, driver_rental.*, avg(comment.on_time)+avg(comment.friendly)+avg(comment.language_skills)+avg(comment.professional) as ranking 
    FROM comment 
    LEFT JOIN driver 
    ON 
    driver.userid = comment.driver_id 
    RIGHT JOIN driver_rental 
    ON 
    driver.email = driver_rental.email 
    WHERE 
    ($driver_rental IS NOT NULL AND $driver_rental != '') AND (car_type_1>=$_SESSION[car_type_1] OR car_type_2>=$_SESSION[car_type_1])"; 

这个想法是不强调任何连接的驱动程序表。 SO而不是第二个左加入使用权加入与driver_rental