Nested Where clause can access global variable but inner join can't - mysql

Question

The below example is just a demonstration of the problem i am facing.
I have two tables A & B on which i want to query. There schemas are as below
create table A
(
id int,
B_id int,
D_id int,
created date
);
create table B
(
id int,
C_id int
);
Table A can have multiple rows for a given B_id.
I insert test data as below :
insert into A(id, B_id, D_id, created) values(2, 1, 0, now());
insert into A(id, B_id, D_id, created) values(3, 1, 0, now());
Now, I want to fetch the newest(whose created is having the highest value) rows in A which have B_id = 1
Now, the problem :
I tried below double inner join which did not work
select * from A
inner join B on A.B_id = B.id
inner join ( select * from A where A.B_id = B.id order by created desc limit 1) as A1 on A.id = A1.id
and A.B_id = 1;
this fails with the error "Unknown column 'B.id' in 'where clause'"
However if i replace the second inner join with a where clause as below, it works :
select * from A
inner join B on A.B_id = B.id
and A.id = ( select id from A where A.B_id = B.id order by created desc limit 1)
and A.B_id = 1;
Why can where clause access the B.id in global scope but inner join can't??

Answer 1

This is a good point. It took me a while to get my head around. First of all I personally would not call it "global" scope. But I've got your point :)
Here is how I understand it. Please correct me if I'm wrong.
First query: I changed your query B.id to 1, so I can run the query correctly. I changed it to the following:
select * from A
inner join B on A.B_id = B.id
inner join ( select * from A where A.B_id = 1 order by created desc limit 1) as A1 on A.id = A1.id
and A.B_id = 1;
After I changed it, I did explain select ... to see how it work. Here is what I've got.
id select_type table type possible_keys key key_len ref rows Extra
--------------------------------------------------------------------------------------------------------
1 PRIMARY <derived2> system null null null null 1 null
1 PRIMARY B ALL null null null null 1 Using where
1 PRIMARY A ALL null null null null 4 Using where; Using join buffer (Block Nested Loop)
2 DERIVED A ALL null null null null 4 Using where; Using filesort
It seems that your subquery select * from A where A.B_id = 1 order by created desc limit 1 is executed during or before INNER JOIN. B.id is not yet available since INNER JOIN hasn't been done yet.
Second query: I did the same explain select ...
id select_type table type possible_keys key key_len ref rows Extra
----------------------------------------------------------------------------------------------------
1 PRIMARY B ALL null null null null 1 Using where
1 PRIMARY A ALL null null null null 4 Using where; Using join buffer (Block Nested Loop)
2 DEPENDENT SUBQUERY A ALL null null null null 4 Using where; Using filesort
As you can see, your subquery is executed after INNER JOIN. Therefore B.id is available.