Joining a selected table to a cross joined table - sql

Question

I have a table, flight_schedule, that consists of a bunch of flight segments. Below I have a Terradata SQL query that creates a list of two segment itineraries between Chicago and Denver. i.e. each row has two flights that eventually get the passenger from Chicago to Denver. For example, the first row contains flight information on a leg from Chicago to Omaha and then a later leg from Omaha to Denver. This query works just fine.
SELECT A.flt_num, A.dprt_sta_cd, A.arrv_sta_cd, A.sch_dprt_dtml, A.sch_arrv_dtml,
B.flt_num, B.dprt_sta_cd, B.arrv_sta_cd, B.sch_dprt_dtml, B.sch_arrv_dtml
FROM
flight_schedule A
CROSS JOIN
flight_schedule B
WHERE
A.dprt_sta_cd = 'Chicago' AND
B.arrv_sta_cd = 'Denver' AND
A.arrv_sta_cd = B.dprt_sta_cd AND
A.sch_arrv_dtml < B.sch_dprt_dtml
ORDER BY B.sch_arrv_dtml;
I have another table, flight_seat_inventory, that consists of seats available in different cabins for each flight number. The query below aggregates total available seats for each flight number. This query is also A-OK.
SELECT flt_num, SUM(seat_cnt) as avail_seats
FROM flight_seat_inventory
GROUP BY flt_num;
I want to combine these two queries with a LEFT JOIN, twice, so that each flight has a corresponding avail_seats value. How can I do this?
For added clarity, I think my desired Select statement looks like this:
SELECT A.flt_num, A.dprt_sta_cd, A.arrv_sta_cd, A.sch_dprt_dtml, A.sch_arrv_dtml, C.avail_seats
B.flt_num, B.dprt_sta_cd, B.arrv_sta_cd, B.sch_dprt_dtml, B.sch_arrv_dtml, D.avail_seats
flight_schedule is HUGE, so I suspect it's more efficient to do the LEFT JOIN after the CROSS JOIN. Again, using Teradata SQL.
Thanks!

Answer 1

I needed to declare the second seats query as a temporary table using a WITH command before I did the LEFT JOIN:
WITH tempSeatsTable AS (
SELECT flt_num, SUM(seat_cnt) as avail_seats
FROM flight_seat_inventory
GROUP BY flt_num
)
SELECT
A.flt_num, A.dprt_sta_cd, A.arrv_sta_cd, A.sch_dprt_dtml, A.sch_arrv_dtml, C.avail_seats
B.flt_num, B.dprt_sta_cd, B.arrv_sta_cd, B.sch_dprt_dtml, B.sch_arrv_dtml, D.avail_seats
FROM
flight_schedule A
CROSS JOIN
flight_schedule B
LEFT JOIN
tempSeatsTable C
ON A.flt_num = C.flt_num
LEFT JOIN
tempSeatsTable D
ON B.flt_num = D.flt_num
WHERE
A.dprt_sta_cd = 'Chicago' AND
B.arrv_sta_cd = 'Denver' AND
A.arrv_sta_cd = B.dprt_sta_cd AND
A.sch_arrv_dtml < B.sch_dprt_dtml
ORDER BY B.sch_arrv_dtml;