Subquery problem

Question

I am trying to use a subquery where rownum =1 and have and order by to get the most recent record. If I do this:
SELECT A1,
(SELECT C1 FROM T2 WHERE C2=T1.C1 AND ROWNUM=1) AS X
FROM T1
I get a random row, so I tried this:
SELECT A1,
SELECT * FROM (SELECT C1 FROM T2 WHERE C2=T1.C1 ORDER BY C3 DESC) WHERE ROWNUM=1)) AS X
FROM T1
and I get an error saying T1.C1 is invalid. Is there a way to do this?
Thanks.

Answer 1

You have one opening bracket and three closing brackets. That's not a good sign ;)
The deeper problem is that a subquery cannot refer to its grandparent; e.g. this is OK:
SELECT dummy
     , ( SELECT t1.dummy
         FROM   ( SELECT dummy c1
                  FROM   dual
                  WHERE  dummy = 'X'  -- uncorrelated
                  ORDER  BY dummy DESC )
         WHERE  rownum = 1 ) AS x
FROM  dual t1;but this is not:
SELECT dummy
     , ( SELECT t1.dummy
         FROM   ( SELECT dummy c1
                  FROM   dual
                  WHERE  dummy = t1.dummy
                  ORDER  BY dummy DESC )
         WHERE  rownum = 1 ) AS x
FROM  dual t1;
ERROR at line 5:
ORA-00904: "T1"."DUMMY": invalid identifier