How to do a full outer join - sql

Question

I set the THEATRES size using THEATRES.NUMOFROWS, THEATRES.NUMOFCOLS and each SEAT.SEATNO is tied to THEATRES.ID and SHOWTIMES.ID is tied to THEATRES.ID and TICKET_ITEMS are tied to SHOWTIMES.ID
I want to display all the values by joining the table using where clause for showtime.id. I know outer join will display null on non-matching but my query only shows one records.
SELECT
SHOWTIMES.ID AS SHOWTIMESID,
SHOWTIMES.THEATREID,
THEATRES.THEATRENAME,
THEATRES.NUMOFROWS,
THEATRES.NUMOFCOLS,
SEAT.SEATNO AS SEATLABEL,
SEAT.ROWID,
SEAT.COLUMNID,
TICKET_ITEMS.SEATNO,
TICKET_ITEMS.TICKETCODE
FROM
SHOWTIMES FULL OUTER JOIN TICKET_ITEMS ON SHOWTIMES.ID =TICKET_ITEMS.SHOWTIMESID
FULL OUTER JOIN THEATRES ON SHOWTIMES.THEATREID = THEATRES.ID
FULL OUTER JOIN SEAT ON SEAT.SEATNO = TICKET_ITEMS.SEATNO
WHERE
SHOWTIMES.ID = 1
;

Answer 1

If you are using any kind of OUTER JOIN and you refer to a column in the WHERE you need to handle NULLs. If you don't, you turn the JOIN into an implicit INNER JOIN. Take this simple example:
WITH T1 AS(
SELECT ID, SomeString
FROM (VALUES(1,'abc'),(2,'def')) V(ID, SomeString)),
T2 AS(
SELECT ID, fID, AnotherString
FROM (VALUES(1,1,'asd'),(2,1,'asdased')) V(ID, fID, AnotherString))
SELECT *
FROM T1
LEFT JOIN T2 ON T1.ID = T2.ID
WHERE T2.AnotherString = 'asd';
You might expect to get 2 rows here, one for where T1.ID is 1 with a joined row and 1 for T1.ID is 2, but with no joined rows. That isn't the case due to the WHERE. The correct solution here would be to move the WHERE to the ON:
WITH T1 AS(
SELECT ID, SomeString
FROM (VALUES(1,'abc'),(2,'def')) V(ID, SomeString)),
T2 AS(
SELECT ID, fID, AnotherString
FROM (VALUES(1,1,'asd'),(2,1,'asdased')) V(ID, fID, AnotherString))
SELECT *
FROM T1
LEFT JOIN T2 ON T1.ID = T2.ID
AND T2.AnotherString = 'asd';
With what you have, you can't do that, as you're using the base table, thus you'll need handle the NULL by changing your WHERE to:
WHERE SHOWTIMES.ID = 1 OR SHOWTIMES.ID IS NULL;

Answer 2

I imagine that your database has proper foreign key relationships set up -- so the theater id in one table is a valid id.
If so, you don't need full join. In fact, it is very rarely needed. I suspect that inner joins are sufficient (but that is the results you are getting).
If you want all theaters with the appropriate shows -- if any -- then use left join and start with the theaters:
select st.ID AS SHOWTIMESID, st.THEATREID,
th.THEATRENAME, th.NUMOFROWS, th.NUMOFCOLS,
s.SEATNO AS SEATLABEL, s.ROWID, s.COLUMNID,
ti.SEATNO, ti.TICKETCODE
from theatres th left join
showtimes st
on st.theatreid = th.id and st.id = 1 left join
ticket_items ti
on st.ID =ti.showtimesid left join
seat s
on s.seatno = ti.seatno