Using values from main query inside a subquery - mysql

Question

I had a problem creating a MySQL query with a subquery.
I wanted to use some data from the main query on the subquery, as many times did.
But this time I wanted to use it in a JOIN and didn't worked. I really want to understand why this happens.
I will show you some examples that works and the one that didn't.
I made this simple structure to reproduce the example:
# table1
id field1
1 *first_value*
2 *another_value*
#table2
id field2
1 *second_value*
Using table1.id on the WHERE of the subquery to get a value, the most typical use for me (I know this can be a join, but i try to show the difference):
SELECT
t1.field1,
(
select t2.field2
FROM table2 as t2
WHERE t2.id = t1.id
) as field2
FROM table1 as t1
WHERE t1.id = '1';
You can use table1.id on the SELECT part too (not much sense in the example, but works):
SELECT
t1.field1,
(
select t1.id as field2
FROM table2 as t2
WHERE t2.id = t1.id
) as field2
FROM table1 as t1
WHERE t1.id = '1';
Now, if you try to use it on a JOIN inside the subquery, then, crashes:
SELECT
t1.field1,
(
select t1.id
FROM table2 as t2
LEFT JOIN table1 as t3 ON t3.id = t1.id
WHERE t2.id = t1.id
) as field2
FROM table1 as t1
WHERE t1.id = '1';
Kernel error: Error( 1054 ) 42S22: "Unknown column 't1.id' in 'on clause'"
Buuut, u can do the JOIN using the field in another subquery changing ON t3.id = t1.id to ON t3.id = (SELECT t1.id) ???
SELECT
t1.field1,
(
select t1.id
FROM table2 as t2
LEFT JOIN table1 as t3 ON t3.id = (SELECT t1.id)
WHERE t2.id = t1.id
) as field2
FROM table1 as t1
WHERE t1.id = '1'
I wonder to know why the third example query doesn't work while all others does.
Can someone explain this, please?
Thank you :)

Answer 1

That's because all elements in the ON clause of a JOIN, must belong the one of the joined tables, so as your t2.id must be equal to t1.id, you can do
SELECT
t1.field1,
(
select t1.id
FROM table2 as t2
LEFT JOIN table1 as t3 ON t3.id = t2.id
WHERE t2.id = t1.id
) as field2
FROM table1 as t1
WHERE t1.id = '1';