Query is executed although sub-query is error?

Question

Hi everyone,
I have a problem want to explained.
My sub-query show an error if it's executed alone: "ORA-00904: invalid column name"
SELECT id FROM table_A WHERE field1 = 'exam' (field1 is not exist)
But when execute with:
SELECT * FROM table_B WHERE id IN (SELECT id FROM table_A WHERE field1 = 'exam')
it's ok, although we don't have any data filtered. I don't known the reason why? Can anyone explain this?
Thanks a lot!

Answer 1

I think you're looking at something like this...
SQL> select deptno from emp where dname = 'SALES';
select deptno from emp where dname = 'SALES'
ERROR at line 1:
ORA-00904: "DNAME": invalid identifier
SQL> select deptno from dept where deptno in (select deptno from emp where dname in 'SALES');
    DEPTNO
        30
SQL> select deptno from dept where deptno in (select deptno from emp where emp.dname in 'SALES');
select deptno from dept where deptno in (select deptno from emp where emp.dname in 'SALES')
ERROR at line 1:
ORA-00904: "EMP"."DNAME": invalid identifier
SQL> ed
Wrote file afiedt.buf
  1* select deptno from dept where deptno in (select deptno from emp where dept.dname in 'SALES')
SQL> /
    DEPTNO
        30In this example, "DNAME" doesn't exist in the emp table, but in the second query it seems to work. However what it's really doing is referencing the "DNAME" column of the outer query. This is proved in the third and fourth query by prefixing the DNAME column with the EMP and DEPT table names to explicitly say where we are expecting the column to come from.