MySQL placement of conditions in on-clauses of multiple outer joins - mysql

Question

As a follow up to In SQL / MySQL, what is the difference between "ON" and "WHERE" in a join statement? and SQL join: where clause vs. on clause - it does matter if a condition is placed in the on-clause vs. the where-clause in an outer join.
However, does it matter which on-clause the condition is placed in when there are multiple outer joins?
For example, could these produce different results?
select * from t1 left join t2 on t1.fid=t2.id and t2.col=val
left join t3 on t2.fid=t3.id;
vs:
select * from t1 left join t2 on t1.fid=t2.id
left join t3 on t2.fid=t3.id and t2.col=val;

Answer 1

Absolutely they are different.
The fisrt query will only have t2 rows that satisfy t2.col=val
The second query will include all t2 rows and only list t3 when t2.col=val

Answer 2

The queries are not equivalent. It is easy to construct a counter example:
create table t1 (id int not null, val int not null);
create table t2 (id int not null, val int not null);
create table t3 (id int not null, val int not null);
insert into t1 (id, val) values (1,1);
insert into t2 (id, val) values (1,1);
insert into t3 (id, val) values (1,1);
select * from t1
left join t2
on t1.id = t2.id
and t2.val = 2
left join t3
on t2.id = t3.id;
+----+-----+------+------+------+------+
| id | val | id | val | id | val |
+----+-----+------+------+------+------+
| 1 | 1 | NULL | NULL | NULL | NULL |
+----+-----+------+------+------+------+
select * from t1
left join t2
on t1.id = t2.id
left join t3
on t2.id = t3.id
and t2.val = 2;
+----+-----+------+------+------+------+
| id | val | id | val | id | val |
+----+-----+------+------+------+------+
| 1 | 1 | 1 | 1 | NULL | NULL |
+----+-----+------+------+------+------+

Answer 3

This is one of the queries:
select *
from t1 left join
t2
on t1.fid = t2.id left join
t3
on t2.fid = t3.id and t2.col = val;
Yes, the results are different. If you were using inner join they would be the same, but the left join changes things -- because the join clause does not do any filtering of rows.
I think the simplest explanation is that the join between t1 and t2 will include all rows from t1 as well as all matching rows from t2 -- even those where t2.col <> val. These remain in the result set, because the next left join does not filter them out.
In fact, the condition t2.col = val in the second on clause does not affect which rows are in the result set. If there is a match, then the row from t3 stays based on the first condition. If there is no match, then the row from t3 is still in the result set -- but the t3 columns will be NULL.
In this version:
select *
from t1 left join
t2
on t1.fid = t2.id and t2.col = val left join
t3
on t2.fid = t3.id;
The first join gets all rows from t1 and only matching rows from t2 where t2.col = val. The third join can then add more rows.
Note: there are definitely situations where the two queries would return the same results. But, the following data would generate different results (assume val = 0):
t1
fid
1
t2
fid col
1 0
1 1
t3
id
1
The query with the condition in the second on clause will return:
1 1 0 1
1 1 1 NULL
The query with the condition in the first on clause will return:
1 1 0 1