mysql加入两个表

Question

table user:
id_u*   f_name   l_name
----------------------
1       andi     mitchel
2       sarah    bench
3       kirsty   larx

table voucher:
id_v*   id_user    id_target
1       1          2
2       2          3

quite confused how to join those table with two foreign keys

$db->query("SELECT * FROM voucher v 
LEFT JOIN user u ON u.id_u = v.id_user
LEFT JOIN user u1 ON u1.id_u = v.id_target
WHERE .... ")

echoing while loop... and returns nothing??

while($r = $q->fetch_array(MYSQLI_ASSOC)) :
   echo  $r['u.f_name'];
   echo  $r['u1.f_name'];
endwhile;

Answer 1

Your JOIN seems absolutely correct. The only issue is that you have joined table user twice, therefore you have columns with same name (like f_name). The database will assign different (but arbitrary) names to these columns. You can override this behaviour with the AS keyword:

$db->query("SELECT v.*
                 , u.f_name AS user_f_name
                 , u.l_name AS user_l_name
                 , ta.f_name AS target_f_name
                 , ta.l_name AS target_l_name
            FROM voucher v 
            LEFT JOIN user u ON u.id_u = v.id_user
            LEFT JOIN user ta ON ta.id_u = v.id_target
            WHERE .... ")

Then:

while($r = $q->fetch_array(MYSQLI_ASSOC)) :
   echo  $r['user_f_name'];
   echo  $r['target_f_name'];
endwhile;

And I think you can replace the LEFT JOINs with (inner) JOINs. Unless you have id_user or id_target values referencing non-existing userids (id_u).