Return max date record previous to start date

Question

Hello,
Can anyone help please?
I have two subqueries, one called tableofchildren, the other called tableofworkers. Tableofchildren identifies the date that the child started a class.
The second query, Tableofworkers, lists workers and their involvement history with the child. A child may have had a history with multiple workers. The worker has an allocatedstartdate indicating when they first started working with the child.
For my purpose, I need to return the workername and allocatedstartdate of the worker involved with the child most recently before or at the time the child started the course.
I've partially accomplished this with the query below. However, due to data quality, the child might not always have a worker allocated (allocatedstartdate) earlier than or equal to the child's course start date. In this case, the query fails and doesn't return the child's record, excluding it from the dataset.
Can anyone suggest a way of amending this query so it acts as a left outer query and returns all the child records and null vlaues where they have no worker start date allocated before or on the course start date?
Thanks! :)
select *
from
select tc.childid,
tc.childname,
tc.enteredcourse,
tw.workername,
tw.allocatedstartdate,
row_number() over ( partition by tc.childid, tc.enteredcourse order by tw.allocatedstartdate desc) rn
from tableofchildren tc, tableofworkers tw
where tc.childid = tw.childid(+)
and tc.enteredcourse >= nvl(tw.allocatedstartdate,add_months(sysdate,-10000))
where rn = 1 desired output
CHILDID CHILDNAME            ENTEREDCOURSEDATE         WORKERNAME           ALLOCATEDSTARTDATE       
C1000   Johnny Rotten        01-APR-11                 Mrs Gerbil           19-AUG-10                
C1256   Doris Dingle         12-AUG-03                 Mrs Pepsi            12-AUG-03                
C3466   Bonny Boy            25-MAR-11                 Mrs Jones            23-FEB-11                
C4567   Casper Ghost         21-MAR-09                                                               
C1245   Doris Dingle         20-NOV-06             create table tableofchildren
(ChildID varchar(6),
ChildName varchar (20),
EnteredCourse date,
LeftCourse date);
insert into tableofchildren (ChildID, ChildName, EnteredCourse, LeftCourse) values ('C1000', 'Johnny Rotten', to_date('01/04/2011','dd/mm/rrrr'), to_date('23/05/2011','dd/mm/rrrr'));
insert into tableofchildren (ChildID, ChildName, EnteredCourse, LeftCourse) values ('C1256', 'Doris Dingle', to_date('12/08/2003','dd/mm/rrrr'), to_date('16/09/2005','dd/mm/rrrr'));
insert into tableofchildren (ChildID, ChildName, EnteredCourse, LeftCourse) values ('C3466', 'Bonny Boy', to_date('25/03/2011','dd/mm/rrrr'), to_date('28/03/2011','dd/mm/rrrr'));
insert into tableofchildren (ChildID, ChildName, EnteredCourse, LeftCourse) values ('C4567', 'Casper Ghost', to_date('21/03/2009','dd/mm/rrrr'), to_date('22/04/2010','dd/mm/rrrr'));
insert into tableofchildren (ChildID, ChildName, EnteredCourse, LeftCourse) values ('C1245', 'Doris Dingle', to_date('20/11/2006','dd/mm/rrrr'), to_date('30/12/2008','dd/mm/rrrr'));
create table tableofworkers
(WorkerID varchar(6),
WorkerName varchar (20),
AllocatedStartDate date,
AllocatedEndDate date,
ChildID varchar(6));
insert into tableofworkers (WorkerID, WorkerName, AllocatedStartDate, AllocatedEndDate, ChildID) values ('W3453', 'Mrs Whatever', to_date('12/05/2009','dd/mm/rrrr'), to_date('13/06/2009','dd/mm/rrrr'), 'C1000');
insert into tableofworkers (WorkerID, WorkerName, AllocatedStartDate, AllocatedEndDate, ChildID) values ('W3442', 'Mr Toad', to_date('14/07/2010','dd/mm/rrrr'), to_date('18/08/2010','dd/mm/rrrr'), 'C1000');
insert into tableofworkers (WorkerID, WorkerName, AllocatedStartDate, AllocatedEndDate, ChildID) values ('W14592', 'Mrs Gerbil', to_date('19/08/2010','dd/mm/rrrr'), NULL, 'C1000');
insert into tableofworkers (WorkerID, WorkerName, AllocatedStartDate, AllocatedEndDate, ChildID) values ('W3442', 'Mrs Pepsi', to_date('12/08/2003','dd/mm/rrrr'), to_date('22/04/2007','dd/mm/rrrr'), 'C1256');
insert into tableofworkers (WorkerID, WorkerName, AllocatedStartDate, AllocatedEndDate, ChildID) values ('W3490', 'Mr Tomato', to_date('12/03/2008','dd/mm/rrrr'), to_date('30/04/2009','dd/mm/rrrr'), 'C3466');
insert into tableofworkers (WorkerID, WorkerName, AllocatedStartDate, AllocatedEndDate, ChildID) values ('W3453', 'Mrs Whatever', to_date('01/06/2009','dd/mm/rrrr'), to_date('30/04/2010','dd/mm/rrrr'), 'C3466');
insert into tableofworkers (WorkerID, WorkerName, AllocatedStartDate, AllocatedEndDate, ChildID) values ('W3457', 'Mrs Jones', to_date('23/02/2011','dd/mm/rrrr'), null, 'C3466');
insert into tableofworkers (WorkerID, WorkerName, AllocatedStartDate, AllocatedEndDate, ChildID) values ('W3453', 'Mrs Jobsworth', to_date('22/11/2006','dd/mm/rrrr'), null, 'C1245');
create table OutputWanted
(ChildID varchar(6),
ChildName varchar (20),
EnteredCourseDate date,
WorkerName varchar(20),
AllocatedStartDate date);
insert into OutputWanted (ChildID, ChildName, EnteredCourseDate, WorkerName, AllocatedStartDate) values ('C1000', 'Johnny Rotten', to_date('01/04/2011','dd/mm/rrrr'), 'Mrs Gerbil', to_date('19/08/2010','dd/mm/rrrr'));
insert into OutputWanted (ChildID, ChildName, EnteredCourseDate, WorkerName, AllocatedStartDate) values ('C1256', 'Doris Dingle', to_date('12/08/2003','dd/mm/rrrr'), 'Mrs Pepsi', to_date('12/08/2003','dd/mm/rrrr'));
insert into OutputWanted (ChildID, ChildName, EnteredCourseDate, WorkerName, AllocatedStartDate) values ('C3466', 'Bonny Boy', to_date('25/03/2011','dd/mm/rrrr'), 'Mrs Jones', to_date('23/02/2011','dd/mm/rrrr'));
insert into OutputWanted (ChildID, ChildName, EnteredCourseDate, WorkerName, AllocatedStartDate) values ('C4567', 'Casper Ghost', to_date('21/03/2009','dd/mm/rrrr'), null, null);
insert into OutputWanted (ChildID, ChildName, EnteredCourseDate, WorkerName, AllocatedStartDate) values ('C1245', 'Doris Dingle', to_date('20/11/2006','dd/mm/rrrr'), null, null);
Edited by: Tiny Penguin on 21-Nov-2011 07:03

Answer 1

Tiny Penguin wrote:
Hi Centinul.
Thanks! That looks like it's working great!
I'm confused though...aside from ANSI joins, how come this works and mine doesn't? You missed a (+) in your WHERE clause:
where tc.childid = tw.childid(+)
and tc.enteredcourse >= nvl(tw.allocatedstartdate,add_months(sysdate,-10000))In this case the second condition is applied AFTER the tables were outer joined filtering out the row from the entire resultset because the enteredcourse date was less than the allocatedstartdate
It probably should have been:
where tc.childid = tw.childid(+)
and tc.enteredcourse >= nvl(tw.allocatedstartdate(+),add_months(sysdate,-10000))In this case the second condition was applied as part of the OUTER join between the two tables, keeping the row with the child ID of 'C1245' from the tableofchildren and discarding it from the tableofworkers (due to LEFT JOIN)
ps...I hate ANSI joins! :DANSI joins rock :)
Edited by: Centinul on Nov 21, 2011 10:52 AM