Reputation: 3442
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;
Upvotes: 0
Views: 511
Reputation: 10342
SELECT * FROM voucher v
LEFT JOIN user u ON u.id_u = v.id_user OR u.id_u = v.id_target
WHERE ....
Upvotes: 0
Reputation: 115650
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 JOIN
s with (inner) JOIN
s. Unless you have id_user
or id_target
values referencing non-existing userids (id_u
).
Upvotes: 2
Reputation: 48179
It looks like you are asking for all people who are in the voucher table regardless of them being in position 1 (user) or position 2 (target)... Then, showing that person's name.
This query does a pre-query of each possible person and their position basis (via WhichPosition).
SELECT STRAIGHT_JOIN
AllVoucherUsers.WhatPosition,
u.*
FROM
( select distinct
v.id_user,
'1' as WhatPosition
from voucher v
union select distinct
v.id_target as id_user,
'2' as WhatPosition
from voucher v
) AllVoucherUsers
join users u
on AllVoucherUsres.id_user = u.id_u
If you only want ONE instance of a given person -- REGARDLESS of their position, just strip out all instances of the "WhatPosition" reference...
SELECT STRAIGHT_JOIN
u.*
FROM
( select distinct
v.id_user
from voucher v
union select distinct
v.id_target as id_user
from voucher v
) AllVoucherUsers
join users u
on AllVoucherUsres.id_user = u.id_u
Upvotes: 1
Reputation: 780
how about: SELECT * FROM voucher JOIN user ON id_user = id_u
Simpler still:
SELECT * FROM voucher, user WHERE id_user = id_u
Upvotes: -1