Select ... in (select ....) retrieves all rows

Question

Hi guys,
I have the following situation:
Table_A
ID NUMBER
NAME VARCHAR2(10)
TABLE_B
ID_B
NAME_B VARCHAR2(10)
By mistake I did the following query:
Select *
from table_a
where id in (select ID from table_b);
My question is: Why the query return all rows from TABLE_A if in the inner select (select ID...) I'm didn't write any of the columns that exist on TABLE_B.?
Hope this is clear.!.
thanks in advance.

Answer 1

select *
from table_a
where id in (select ID from table_b);The subquery will attempt to resolve the column named "ID" first within the subquery itself, then (if it doesn't find it) in the parent query. Since there is no ID column in table_b, the query actually becomes something like
select *
from table_a a
where a.id in (select a.id from table_b);
in other words
select *
from table_a
where id = id
which is equivalent to
select *
from table_a
where id IS NOT NULL
HTH