Why query does not fail with nonexistent column in subquery?

Question

I misspelled in my query and faced with MySQL\'s strange behaviour.

create table aaa (id bigint auto_increment primary key, 
                  amount int not nul         


        

Answer 1

This query:

select sum(amount)
from aaa
where id not in (select id from bbb);

Is interpreted as:

select sum(aaa.amount)
from aaa
where aaa.id not in (select aaa.id from bbb);

because bbb.id does not exist. When writing SQL, I suggest that you always use table aliases. The query that you thought you were writing:

select sum(aaa.amount)
from aaa
where aaa.id not in (select bbb.id from bbb);

would generate the error you expect.

Answer 2

If you created the bbb table like this:

create table bbb (aaa_id bigint not null, 
                  ^^^^^^

why would you use

select sum(amount) from aaa where id not in (select id from bbb);
                                                    ^^

then instead? The DB's not telepathic, it can't read your mind. If you tell it to use field x in a table, then it's going to need x to actually exist, and won't randomly pick some OTHER field.

The subquery COULD "reach out" and pick up field names from the outer/parent query, if tell the DB to do so, e.g.

SELECT id
FROM aaa
WHERE 
   id in (SELECT ... FROM bbb WHERE aaa.id = bbb.somefield)