Join these 2 tables to get this specific output? - sql

Question

I am using SQL Server 2014. I am working on a query where I want to extract information from 2 specific tables to create my final output.
An extract of the 2 tables (Rebookings and ResaList) are given below.
Rebookings Table (each CancelledID has its corresponding RebookingID):
CancelledID RebookingID
102 541
250 351
129 800
...
ResaList Table:
ID Property ArrivalDate RN
100 X 2020-05-22 9
102 X 2020-03-05 7
250 D 2020-04-12 10
129 E 2020-03-15 8
351 D 2020-09-23 5
541 X 2020-06-01 7
800 E 2020-07-11 8
...
Here is my desired output:
ID Property ArrivalDate RN RebookingID Rebooking_ArrivalDate Rebooking_RN Tag
102 X 2020-03-05 7 541 2020-06-01 7 Cancelled
250 D 2020-04-12 10 351 2020-09-23 5 Re-booked
129 E 2020-03-15 8 800 2020-07-11 8 Re-booked
This is what I have done so far:
USE [MyDatabase]
select
a.[ID],
a.[Property],
a.[Arrival Date],
a.[RN],
b.[RebookingID],
(CASE WHEN a.[ID] in (SELECT [CancelledID] FROM [Rebookings]) THEN 'Re-booked'
ELSE 'Cancelled'
END) as [Tag]
from [ResaList] a
LEFT JOIN [Rebookings] b on b.[CancelledID] = a.[ID]
where a.[ID] in (SELECT [CancelledID] FROM [Rebookings])
GROUP BY a.[ID], a.[Property], a.[ArrivalDate], b.[RebookingID]
I am stuck at how to bring Rebooking_ArrivalDate and Rebooking_RN into the above output. Any help would be appreciated.

Answer 1

I have tried this in SQL Server.
DECLARE #cancelled Table (cancelledId int, rebookingId int)
INSERT into #cancelled values
(102 , 541 ),
( 250 , 351 ),
( 129 , 800 );
DECLARE #ResaList TABLE(Id int, property CHAR(1), ArrivalDate date, RN int)
INSERT INTO #ResaList values
(100 ,'X','2020-05-22', 9),
(102 ,'X','2020-03-05', 7),
(250 ,'D','2020-04-12', 10),
(129 ,'E','2020-03-15', 8),
(351 ,'D','2020-09-23', 5),
(541 ,'X','2020-06-01', 7),
(800 ,'E','2020-07-11', 8);
SELECT r.Id, r.Property, r.ArrivalDate, r.rn,
rebooking.RebookingId
, rebooking.Rebooking_ArrivalDate
, rebooking.Rebooking_RN
, CASE WHEN rebooking.RebookingId IS NOT NULL THEN 'Re-booked' ELSE 'cancelled' end as tag
FROM #ResaList as r
OUTER APPLY (SELECT rc.Id, rc.ArrivalDate, rc.RN
FROM #cancelled as c
INNER JOIN #ResaList AS rc
ON rc.Id = c.rebookingId
WHERE c.cancelledId = r.Id) as rebooking(RebookingId, Rebooking_ArrivalDate, Rebooking_RN)
+-----+----------+-------------+----+-------------+-----------------------+--------------+-----------+
| Id | Property | ArrivalDate | rn | RebookingId | Rebooking_ArrivalDate | Rebooking_RN | tag |
+-----+----------+-------------+----+-------------+-----------------------+--------------+-----------+
| 100 | X | 2020-05-22 | 9 | NULL | NULL | NULL | cancelled |
| 102 | X | 2020-03-05 | 7 | 541 | 2020-06-01 | 7 | Re-booked |
| 250 | D | 2020-04-12 | 10 | 351 | 2020-09-23 | 5 | Re-booked |
| 129 | E | 2020-03-15 | 8 | 800 | 2020-07-11 | 8 | Re-booked |
| 351 | D | 2020-09-23 | 5 | NULL | NULL | NULL | cancelled |
| 541 | X | 2020-06-01 | 7 | NULL | NULL | NULL | cancelled |
| 800 | E | 2020-07-11 | 8 | NULL | NULL | NULL | cancelled |
+-----+----------+-------------+----+-------------+-----------------------+--------------+-----------+

Answer 2

Using select in an inner query to find out existing records is very bad way to do this. On larger tables you will suffer a lot.
Here is a compact and efficient way-
select
a.ID,
a.Property,
a.Arrival Date,
a.RN,
b.RebookingID,
b.Rebooking_ArrivalDate,
b.Rebooking_RN,
coalesce(b.tag,'Cancelled')as tag
from ResaList a left join
(
select *, 'Re-booked' as tag from Rebookings
) b
on a.ID=b.CancelledID

Answer 3

You can select the same table twice by using aliases.
SELECT
*
,(CASE WHEN b.[ID] IS NOT NULL THEN 'Re-booked'
ELSE 'Cancelled'
END) as [Tag]
FROM ResaList AS a
LEFT JOIN Rebookings ON a.ID = Rebookings.CancelledID
LEFT JOIN ResaList AS b ON b.ID = Rebookings.RebookingID