Help! one to many relationship table and display output as exists

Question

I have two tables:
Table1: LOCATION with field locn_id
Table2: TRACKING with field from_locn and to_locn
Example
locn_id from_locn to_locn
26001A 26001A 26002A
26002A 26001A 26001B
26001B 26001A 26001C
26001C
I am linking locn_id to from_locn.
I want to display output as:
If locn_id exists in from_locn (I dont care how many records), display Y. If locn_id does not exist then display N.
So in above case, output should be
26001A Y
26002A N
26001B N
26001C N

Answer 1

Here is my try:
create table LOCATION(locn_id   VARCHAR2(10));
create table TRACKING(locn_id   VARCHAR2(10), from_locn  VARCHAR2(10), to_locn  VARCHAR2(10));
insert into LOCATION values('26001A');
insert into LOCATION values('26002A');
insert into LOCATION values('26001B');
insert into LOCATION values('26001C');
insert into TRACKING values('26001A', '26001A', '26002A');
insert into TRACKING values('26002A', '26001A', '26001B');
insert into TRACKING values('26001B', '26001A', '26001C');
insert into TRACKING values('26001C', '',       ''      );
select l.locn_id,
(select decode(count(*), 0, 'N', 'Y') from tracking t where l.locn_id = t.from_locn ) exists_flag
from   location l
LOCN_ID    E
26001A     Y
26002A     N
26001B     N
26001C     N
4 rows selected.Edited by: Thomas Morgan on May 31, 2013 3:43 PM