Thread: Conditional left join
I have the following table structure on Postgres 8.4 :
STRUCTURE: tblunit
unit_id [integer]
unit_location [character varying]
DATA:
1,'location1'
2,'location2'
3,'location3'
STRUCTURE: tbloperator
operator_id [integer]
operator_name [character varying]
DATA:
1,'operator1'
2,'operator2'
3,'operator3'
4,'operator4'
5,'operator5'
6,'operator6'
STRUCTURE: tbloperatorschedule
operator_schedule_id [bigint]
operator_id [integer] {Foreign key tbloperator->operator_id}
schedule_start_time [timestamp without time zone]
schedule_end_time [timestamp without time zone]
unit_id [bigint] {Foreign key tblunit->unit_id}
DATA:
1,1,'2011-12-01 01:00:00','2011-12-01 02:00:00',1
2,5,'2011-12-01 02:30:00','2011-12-01 04:50:00',1
3,2,'2011-12-01 04:55:00','2011-12-01 10:20:00',1
4,1,'2011-12-01 03:00:00','2011-12-01 05:00:00',2
5,3,'2011-12-01 05:30:00','2011-12-01 09:50:00',2
6,4,'2011-12-01 09:55:00','2011-12-01 13:20:00',2
7,6,'2011-12-01 14:00:00','2011-12-01 18:00:00',2
8,5,'2011-12-01 06:30:00','2011-12-01 14:50:00',3
9,2,'2011-12-01 14:55:00','2011-12-01 20:20:00',3
STRUCTURE: tbldata
data_id [bigint]
event_time [timestamp without time zone]
data_text [character varying]
unit_id [bigint] {Foreign key tblunit->unit_id}
DATA:
1,'2011-12-01 02:30:00','abc',1
2,'2011-12-01 06:28:00','abc',2
3,'2011-12-01 11:10:00','abc',3
4,'2011-12-01 21:30:00','abc',3
I am trying (through conditional left join?) to fetch all records of tbldata and the operator name from tbloperators who was operating the unit at event time. If no operator was present, it should return null.
Resulting set:
1,'2011-12-01 02:30:00','abc',1,operator5
2,'2011-12-01 06:28:00','abc',2,operator3
3,'2011-12-01 11:10:00','abc',3,operator5
4,'2011-12-01 21:30:00','abc',3,NULL
The sql query to create the tables and sample data is attached or can be seen at http://pastebin.com/Fy2t3H9S . Is it possible to get a condition within a left join or any other way to fetch the desired data.
Amitabh
Attachment
On 1 December 2011 13:16, Amitabh Kant <amitabhkant@gmail.com> wrote:
> I am trying (through conditional left join?) to fetch all records of tbldata
> and the operator name from tbloperators who was operating the unit at event
> time. If no operator was present, it should return null.
I think you want something akin to:
SELECT *
FROM tbldata AS a
LEFT JOIN (tbloperatorschedule INNER JOIN tbloperator USING (operator_id)) AS b
ON (a.unit_id = b.unit_id AND
a.event_time BETWEEN b.schedule_start_time AND b.schedule_end_time
)
--
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Cut the trees and you'll see there is no forest.