How Sequence Matters in Laravel query builder for mysql - mysql

Question

The complied query has different sequence from the code.
The query works but I don't know why it works!
Here is the business:
I get the roomId by the $userId, and then I get all the playerId by the roomId
Steps:
in Table room_player_relations, I get a row where playerId = $userId.
in Table room_player_relations, I get all the rows where roomId = the roomId of the row found in point 1.
select playerId from the rows found in point 2.
Here is the code:
$players = DB::table('room_player_relations as t1')
->where('t1.playerId',$userId)
->join('room_player_relations as t2','t1.roomId','=','t2.roomId')
->select('t2.playerId')
->get();
Here is the query seen in the debugbar:
select `t2`.`playerId` from `room_player_relations` as `t1`
inner join `room_player_relations` as `t2`
on `t1`.`roomId` = `t2`.`roomId`
where `t1`.`playerId` = '1'
It looks stupid because it join 2 the same tables together, the where is put afterward. But it works. Please tell me why? Thanks!

Answer 1

I find out INNER JOIN-ing two table is not sticking two tables together, but matching all the possible ways of the matching key (roomIdin my example). I can actually see the result of INNER JOIN by the following query:
select * from `room_player_relations` as `t1`
inner join `room_player_relations` as `t2`
on `t1`.`roomId` = `t2`.`roomId`
playerId roomId playerId roomId
2 2 2 2
3 2 2 2
1 2 2 2
2 2 3 2
3 2 3 2
1 2 3 2
4 1 4 1
5 1 4 1
4 1 5 1
5 1 5 1
2 2 1 2
3 2 1 2
1 2 1 2