Multiple joins with multiple conditions with multiple tables - mysql

Question

at first my tables:
game
+----+--------------+
| id | game |
+----+--------------+
| 1 | Game1 |
| 2 | Game2 |
| 4 | Game4 |
+----+--------------+
group_game
+---------+----------+
| game_id | group_id |
+---------+----------+
| 1 | 33 |
| 1 | 45 |
| 4 | 33 |
+---------+----------+
groups
+----+------------+----
| id | group_name | ...
+----+------------+----
| 33 | Group33 | ...
| 45 | Group45 | ...
+----+------------+----
users
+---------+----------+----
| user_id | username | ...
+---------+----------+----
| 1 | User1 | ...
| 2 | User2 | ...
+---------+----------+----
users_groups
+---------+----------+
| user_id | group_id |
+---------+----------+
| 1 | 33 |
| 1 | 45 |
| 2 | 45 |
+---------+----------+
What I want to do
Now I want to check wether the current user is in a group which plays "Game4" and if yes the output should be the id and the name of the group.
the current user is "User1" with the ID 1 (table users)
"User1" is in a group with the ID 33 (table users_groups)
The Group-ID 33 belongs to "Group33" (table groups)
The Group with the ID 33 plays the Game with the ID 4 (table group_game)
The Game with the ID belongs to the game "Game4" (table game)
CONCLUSION: Yes, the user is in a group which plays Game4, so output the name of the group ("Group33")
My current code for that (which gives me no rows)
$user_id = $_SESSION["user_id"];
$Game4= "Game4";
$gruppen_dayz = $db->prepare("
SELECT g.group_id, g.group_name
FROM groups g
LEFT JOIN users_groups ug
ON g.group_id = ug.group_id
LEFT JOIN group_game gg
ON g.group_id = gg.group_id
LEFT JOIN game ga
ON ga.id = gg.game_id
WHERE ga.game = ? AND ug.user_id = ?
");
$gruppen_dayz->bind_param('ii', $Game4, $user_id);
I don't know exactly how I should build this query :/

Answer 1

Your where clause is being applied after the joins generate the dataset, negating the left join aspect of your joins. To solve this move the criteria to the join itself so the limits are applied before the joins. Otherwise, the NULL values generated in your left joins are then excluded by your where clause.
There may be other elements as well, this is just the first component I saw.
$user_id = $_SESSION["user_id"];
$Game4= "Game4";
$gruppen_dayz = $db->prepare("
SELECT g.group_id, g.group_name
FROM groups g
LEFT JOIN users_groups ug
ON g.group_id = ug.group_id
AND ug.user_id = ?
LEFT JOIN group_game gg
ON g.group_id = gg.group_id
LEFT JOIN game ga
ON ga.id = gg.game_id
AND ga.game = ?
");
$gruppen_dayz->bind_param('ii', $Game4, $user_id);
---UPDATE ----
Upon further investigation I believe your joins are wrong. Groups doesn't have a group_ID field according to your table structure. Walking though the rest now...
SELECT g.group_id, g.group_name
FROM groups g
LEFT JOIN users_groups ug
ON g.id = ug.group_id
LEFT JOIN group_game gg
ON g.id = gg.group_id
LEFT JOIN game ga
ON ga.id = gg.game_id
WHERE ga.game = ? AND ug.user_id = ?
I might rewrite it as...
SELECT G.Id, G.Group_name
FROM USERS_GROUPS UG
INNER JOIN GROUPS G
on UG.Group_ID = G.ID
INNER JOIN GROUP_GAME GG
on GG.Group_ID = G.ID
INNER JOIN Game GA
on GA.ID = GG.Game_ID
WHERE ga.game = ? AND ug.user_id = ?
I see no value or need for left joins based on your criteria.