联接表上的PHP错误说不是唯一表或别名

Question

I don't know how to fix this thing a little help would be good.

Error : table/alias is not unique.

Why do i got this error? I believe why joins are correct.Please help

 $q = "SELECT * FROM `curriculum` "
   . "INNER JOIN `subject` ON `curriculum`.`subject`=`subject`.`sub_id` "
   . "LEFT JOIN `subject` ON `curriculum`.`cur_pr`=`subject`.`sub_id` "
   . "WHERE `course`=:cid and `cur_year`=1";

Answer 1

Change your Code to

 $q = "SELECT * FROM `curriculum` AS A "
   . "INNER JOIN `subject` AS B ON `A`.`subject`=`B`.`sub_id` "
   . "LEFT JOIN `subject` AS C ON `A`.`cur_pr`=`C`.`sub_id` "
   . "WHERE `course`=:cid and `cur_year`=1";

If you want to join Same tables again use Table Aliases Here AS is used to this purpose

Answer 2

You get an error that says table/alias is not unique because this line code:

. "LEFT JOIN subject ON curriculum . cur_pr = subject . sub_id "

It don't know what subject is belong to what table ( first one or the second one). You can try this code above

$q = "SELECT * FROM `curriculum` AS cu "
   . "INNER JOIN `subject` AS su1 ON `cu`.`subject`=`su1`.`sub_id` "
   . "LEFT JOIN `subject` AS su2 ON `cu`.`cur_pr`=`su2`.`sub_id` "
   . "WHERE `cu.course`=:cid and `cu.cur_year`=1";