php - MySQL总是返回期望参数1为mysqli_result，给定为boolean

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mysql_fetch_array()/mysql_fetch_assoc()/mysql_fetch_row()/mysql_num_rows etc… expects parameter 1 to be resource or result
                                
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我有3张桌子。在表_1和表_2中，我有一个名为plevel的字段。第三张桌子是加入他们并且是。
cars_user并有2列-table1_plevel和table2_plevel
我要选择的查询是

$q = mysqli_query($con, "SELECT * FROM `cars` AS c
      LEFT JOIN `cars_user` AS c2u ON c.cars_plevel = c2u.car_plevel
      LEFT JOIN `users` AS u ON c2u.user_plevel = u.users_plevel");

当我运行它时，我知道该列为“未知”，但我知道该列在那里。

 #1054 - Unknown column 'c.cars_plevel' in 'on clause'

我在cars_user中也引用了cars.plevel和users.plevel

ALTER TABLE `cars_user` ADD FOREIGN KEY ( `cars_plevel` )
     REFERENCES `app`.`cars` (`plevel`) ON DELETE RESTRICT ON UPDATE RESTRICT;
ALTER TABLE `cars_user` ADD FOREIGN KEY ( `user_plevel` )
     REFERENCES `app`.`users` (`plevel`) ON DELETE RESTRICT ON UPDATE RESTRICT;

这里有什么问题？

最佳答案

我喜欢错误说您没有c.cars_plevel您没有c.plevel并且您没有u.users_plevel您具有u.plevel或换句话说：

$q = mysqli_query($con, "SELECT * FROM `cars` AS c
  LEFT JOIN `cars_user` AS c2u ON c.plevel = c2u.car_plevel
  LEFT JOIN `users` AS u ON c2u.user_plevel = u.plevel");